diff --git a/build/pnltri.js b/build/pnltri.js
index 5cddcc0..aa69f39 100644
--- a/build/pnltri.js
+++ b/build/pnltri.js
@@ -2183,7 +2183,17 @@ PNLTRI.MonoTriangulator.prototype = {
 		while ( (frontVert != endVert) || (vertBackLogIdx > 1) ) {
 			if (vertBackLogIdx > 0) {
 				// vertBackLog is not empty
-				if ( PNLTRI.Math.ptsCrossProd( frontVert, vertBackLog[vertBackLogIdx-1], vertBackLog[vertBackLogIdx] ) > 0 ) {		// TODO !!
+				var angle = PNLTRI.Math.ptsCrossProd( frontVert, vertBackLog[vertBackLogIdx-1], vertBackLog[vertBackLogIdx] );		// TODO !!
+				if ( Math.abs(angle) <= PNLTRI.Math.EPSILON_P ) {
+					// co-linear
+					if ( PNLTRI.Math.compare_pts_yx( frontVert, vertBackLog[vertBackLogIdx] ) != PNLTRI.Math.compare_pts_yx( vertBackLog[vertBackLogIdx], vertBackLog[vertBackLogIdx-1] ) ) {
+//						console.log("triangulate_monotone_polygon: colinear", frontVert.x - vertBackLog[vertBackLogIdx].x, frontVert.y - vertBackLog[vertBackLogIdx].y,
+//													vertBackLog[vertBackLogIdx].x - vertBackLog[vertBackLogIdx-1].x, vertBackLog[vertBackLogIdx].y - vertBackLog[vertBackLogIdx-1].y,
+//													frontVert, vertBackLog[vertBackLogIdx], vertBackLog[vertBackLogIdx-1] );
+						angle = 1;		// co-linear-reversal => create triangle
+					}
+				}
+				if ( angle > 0 ) {
 					// convex corner: cut if off
 					this.polyData.addTriangle( vertBackLog[vertBackLogIdx-1], vertBackLog[vertBackLogIdx], frontVert );
 					vertBackLogIdx--;
diff --git a/build/pnltri.min.js b/build/pnltri.min.js
index df4ee66..f63538b 100644
--- a/build/pnltri.min.js
+++ b/build/pnltri.min.js
@@ -1,21 +1,21 @@
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+function w(b,c){function e(a,b){return Math.abs(a.x-b.x)<r.d.k&&Math.abs(a.y-b.y)<r.d.k}function f(a,b,c){if(Math.abs(r.d.P(b,a,c))>r.d.k)return!1;var d;Math.abs(a.y-b.y)<r.d.k?(d=b.x,a.x<c.x?(b=a.x,a=c.x):(b=c.x,a=a.x)):(d=b.y,a.y<c.y?(b=a.y,a=c.y):(b=c.y,a=a.y));return b-d<r.d.k&&d-a<r.d.k}for(var a=[],h,g,d,m=0;m<c.length;m++)h=G(b,c[m].x,c[m].y),g=!0,d=a.length-1,0<=d&&(e(h,a[d])?g=!1:0<d&&f(a[d-1],a[d],h)&&a.pop()),g&&a.push(h);d=a.length-1;0<d&&e(a[d],a[0])&&(a.pop(),d--);1<d&&(f(a[d-1],a[d],
+a[0])&&(a.pop(),d--),1<d&&f(a[d],a[0],a[1])&&a.shift());return a}function x(b,c,e){return{S:b.N,c:c,j:e,C:1==r.d.H(e,c),n:null,m:null,I:null,J:null,v:!1,K:null,L:null,h:null,g:null,U:!1}}function G(b,c,e){c={id:b.da.length,x:c,y:e,w:[]};b.da.push(c);return c};function H(b){this.i=b}H.prototype={};function I(b,c,e,f){this.u=b?b:{x:Number.POSITIVE_INFINITY,y:Number.POSITIVE_INFINITY};this.l=c?c:{x:Number.NEGATIVE_INFINITY,y:Number.NEGATIVE_INFINITY};this.p=e;this.r=f;this.depth=-1;this.V=!1}I.prototype={};function J(b){var c=new I(b.u,b.l,b.p,b.r);c.e=b.e;c.f=b.f;c.a=b.a;c.b=b.b;c.t=b.t;return c}function K(b){this.A=b;b.t=this}K.prototype={};
+function L(b){var c=new I(null,null,null,null);this.B=[];N(this,c);this.root=new K(c);if(b)for(b=b.q,c=0;c<b.length;c++)b[c].I=b[c].J=this.root,b[c].v=!1}L.prototype={M:function(){for(var b,c=0,e=this.B.length;c<e;c++)if(b=this.B[c],1==b.depth%2&&!b.V&&(!b.e&&!b.f||!b.a&&!b.b)&&b.p)return b;return null}};
+function O(b,c){function e(){var a=l.e||l.f;a.a&&a.b?l==a.a?(n.e=null,a.a=q):(q.f=null,a.b=n):(n.e=null,n.f=a,a.b=n)}function f(a){l.l==t.l?(d?l.a?(a.e=q,q.a=a,n.b=null):(a.f=n,q.a=null,n.b=a):(a.e=q,a.f=n,q.a=n.b=a),q.b=n.a=null):(a.e&&a.f&&(a.e==l?(a.D=a.f,a.ba=!0):(a.D=a.e,a.ba=!1)),a.e=q,a.f=n,q.b=n.a=a,q.a=n.b=null)}function a(){var a;if(l.l==t.l&&d)l.a.e=q,l.b.f=n,q.a=l.a,n.b=l.b,a=q.b=n.a=null;else{l.a.e=q;l.b.f=n;var b=P(c,l.l);if(0<b)a=l.b,l.b.e=q,q.a=l.a,n.b=null;else if(0>b)a=l.a,l.a.f=
+n,n.b=l.b,q.a=null;else{a=l.a.r;var e=a.C,b=e?a.c:a.j,b=P(c,b);0<b?(a=l.b,l.b.e=q,q.a=l.a,n.b=null):0>b?(a=l.a,l.a.f=n,n.b=l.b,q.a=null):(a=e?a.m:a.n,b=e?a.j:a.c,b=P(c,b),0<b?(a=l.b,l.b.e=q,q.a=l.a,n.b=null):(a=l.a,l.a.f=n,n.b=l.b,q.a=null))}q.b=n.a=a}return a}Q(c);var h,g,d,m,k,p;c.C?(h=c.c,m=c.j,g=c.I,k=c.J,d=c.n.v,p=c.m.v):(h=c.j,m=c.c,g=c.J,k=c.I,d=c.m.v,p=c.n.v);p||(p=R(b,k,m,!1),k==g&&(g=p),k=p);k=k.A;if(k.e||k.f)if(k.u!=m)console.log("ERR add_segment: trFirstHigh != segHigh: ",k);else{d||(g=
+R(b,g,h,!0));var t=g.A,l=k,q,n,y,u;for(h=b.B.length+2;l;){if(0>--h){console.log("ERR add_segment: infinite loop",l,c,b);return}if(!l.a&&!l.b){console.log("ERR add_segment: missing successors",l,c,b);return}g=l.t;g.s=c;g.A=null;u&&u.r==l.r?(q=l,n=u,n.l=l.l,g.left=new K(q),g.right=u.t):(y&&y.p==l.p?(n=l,q=y,q.l=l.l,g.left=y.t):(q=l,n=ba(b,l),g.left=new K(q)),g.right=new K(n));l.e&&l.f?l.D?(l.ba?(n.e=l.f,n.f=l.D,n.e.a=n,n.f.b=n):(q.f=l.e,q.e=l.D,q.e.a=q,q.f.b=q),q.D=n.D=null):l.u==k.u?(n.f.b=n,q.f=n.e=
+null):n==l?(n.e=n.f,n.f=null,n.e.a=n):(q.f=q.e,q.e=null):e();l.a&&l.b?g=a():(g=l.a?l.a:l.b,f(g));q.r&&(q.r.K=n);n.p&&(n.p.L=q);q.r=n.p=c;c.K=q;c.L=n;l.l!=t.l?(y=q,u=n,l=g):l=null}c.v=!0}else console.log("ERR add_segment: missing trFirst.uX: ",k)}
+function S(b,c){if(c)var e=b.c,f=b.j,a=b.I;else e=b.j,f=b.c,a=b.J;for(var h;a;)if(a.Y)h=e==a.Y?f:e,h=r.d.H(h,a.Y),a=-1==h?a.left:a.right;else if(a.s)e==a.s.c||e==a.s.j?Math.abs(e.y-f.y)<r.d.k?a=f.x<e.x?a.left:a.right:(h=P(a.s,f),a=0<h?a.left:0>h?a.right:e==a.s.c?a.right:a.left):(h=P(a.s,e),0<h?a=a.left:0>h?a=a.right:(h=P(a.s,f),0<h?a=a.left:0>h?a=a.right:(h=P(a.s,c?b.n.c:b.m.j),a=0<h?a.left:a.right)));else{a.A||console.log("ptNode: unknown type",a);c?b.I=a:b.J=a;break}}
+function Q(b){S(b,!0);S(b,!1)}function P(b,c){var e,f;f=b.c.x-c.x;var a=b.j.x-c.x;e=Math.abs(b.c.y-c.y)<r.d.k;if(Math.abs(b.j.y-c.y)<r.d.k){if(e)return 0;e=a}else if(e)e=f,f=a;else return b.C?r.d.P(b.c,b.j,c):r.d.P(b.j,b.c,c);return Math.abs(e)<r.d.k?Math.abs(f)<r.d.k?0:f:e}function R(b,c,e,f){var a=c.A;if(a.u==e||a.l==e)return c;var h=J(a);a.l=h.u=e;a.a=h;h.e=a;a.b=h.f=null;h.a&&(h.a.e=h);h.b&&(h.b.f=h);N(b,h);c.Y=e;c.A=null;c.right=new K(a);c.left=new K(h);return f?a.t:h.t}
+function ba(b,c){var e=J(c);N(b,e);return e}function N(b,c){c.ka=b.B.length;b.B.push(c)}function X(b){this.i=b;this.$=new L(this.i)}X.prototype={M:function(){return this.$.M()}};function Y(b){this.i=b;this.aa=this.R=null}Y.prototype={};
+function ca(b,c,e){var f=null,a=null;function h(){var a;return(a=g.pop())?(d=a[0],m=a[1],k=a[2],p=a[3],!0):!1}var g=[],d,m,k,p,t;null==f&&(a=!0,e.e?f=!0:e.a?f=!1:(a=!1,f=e.f?!0:!1));for(e&&g.push([e,f,a,c]);h();)if(!d.V){d.V=!0;if(!d.p||!d.r){console.log("ERR alyTrap: lseg/rseg missing",d);break}m?k?(c=d.e,e=d.f,f=d.a,a=d.b):(c=d.f,e=d.e,f=d.b,a=d.a):k?(c=d.a,e=d.b,f=d.e,a=d.f):(c=d.b,e=d.a,f=d.f,a=d.e);if(e||a)t=D(b.i,p,d.l,d.u,k);a&&g.push([a,m,!k,t]);e&&g.push([e,!m,!k,t]);f&&g.push([f,m,k,p]);
+f||a||c&&g.push([c,!m,k,p])}};function Z(b){this.i=b}Z.prototype={};function $(){this.O=null}
+$.prototype={Z:function(){this.O=null},T:function(){return this.O?this.O.T():null},ja:function(b,c){this.Z();if(!b||0==b.length)return[];var e=new v(b),f=c?!1:1==e.N;if(f){f=new H(e);a:{var a=f.i,h=a.q[0],g=h,d=h,m=0;do m+=(d.c.x-d.j.x)*(d.c.y+d.j.y),d=d.m;while(d!=h);if(0>m){do g.h=g.m,g=g.g=g.n;while(g!=h);a.G[0]=!1}else{do g.h=g.n,g=g.g=g.m;while(g!=h)}for(h=a=h;a.g!=a.h;){b:{var g=a.h.c.x,d=a.h.c.y,m=a.c.x,k=a.c.y,p=a.g.c.x,t=a.g.c.y,l=p-m,q=t-k,n=g-p,y=d-t,u=m-g,M=k-d;if(r.d.k>u*q-l*M)g=!1;else{for(var aa=
+a.h.h,C=a.g;C!=aa;){var C=C.g,z=C.c.x,A=C.c.y,T=z-g,U=A-d;if(0!=T||0!=U){var V=z-m,W=A-k;if(0!=V||0!=W)if(z-=p,A-=t,(0!=z||0!=A)&&l*W-q*V>=r.d.F&&u*U-M*T>=r.d.F&&n*A-y*z>=r.d.F){g=!1;break b}}}g=!0}}if(g)B(f.i,a.h.c,a.c,a.g.c),a.h.g=a.g,a.g.h=a.h,h=a=a.g;else if(a=a.g,a==h){f=!1;break a}}f=!0}}if(!f){f=new Y(e);f.R=new X(f.i);h=f.R;a=h.i.q.concat();r.d.fa(a);g=0;d=h.i.N;if(1!=d)for(m=Array(d),k=a.concat(),p=0;p<k.length;p++)t=k[p].S,m[t]?a[d++]=k[p]:(a[g++]=k[p],m[t]=!0);g=a.length;d=h.$;m=0;for(k=
+g;m<g;){k=Math.log(k)/Math.LN2;for(p=1<k?Math.floor(g/k):g;m<p;m++)O(d,a[m]);for(p=m;p<g;p++)Q(a[p])}var h=h.i,d=[d.B[0]],m=[],s,p=0;do{for(t=1==p%2;k=d.pop();)-1==k.depth&&(k.depth=p,k.e&&d.push(k.e),k.f&&d.push(k.f),k.a&&d.push(k.a),k.b&&d.push(k.b),(s=k.p)&&-1==s.K.depth&&m.push(s.K),s=k.r)&&(-1==s.L.depth&&m.push(s.L),s.C!=t&&(h.G[s.S]=!1));d=m;m=[];p++}while(0<d.length);for(p=0;p<g;p++)a[p].K=a[p].L=null;f.aa=f.R.M();s=f.i;for(h=0;h<s.q.length;h++)a=s.q[h],s.G[a.S]?(a.h=a.n,a.g=a.m,a.c.w.push({Q:a,
+X:a.j})):(a=a.m,a.h=a.m,a.g=a.n,a.c.w.push({Q:a,X:s.q[h].c}));for(s=f.aa;s;)a=ca,h=f,g=f.i,d=g.o.length,g.o[d]=s.p,a(h,d,s),s=f.R.M();s=f.i;m=[];for(k=0;k<s.o.length;k++){f=a=s.o[k];g=d=f.c;f.U=!0;f=f.g;for(p=!1;(h=f.c)!=g;){if(f.U){p=!0;break}else f.U=!0;1==r.d.H(h,d)&&(d=h,a=f);f=f.g}p||m.push(a)}s.o=m;s=f=new Z(e);f=s.i.o;s.i.W=[];for(a=0;a<f.length;a++)if(d=f[a],h=d.h,g=d.g,g.g==h)B(s.i,d.c,g.c,h.c);else if(h=s,g=d.g,d=d.c,m=[g.c],k=0,g=g.g,p=g.c,p!=d){for(;p!=d||1<k;)if(0<k)if(t=r.d.P(p,m[k-
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diff --git a/src/MonoTriangulator.js b/src/MonoTriangulator.js
index 94a6893..8d79923 100644
--- a/src/MonoTriangulator.js
+++ b/src/MonoTriangulator.js
@@ -83,7 +83,17 @@ PNLTRI.MonoTriangulator.prototype = {
 		while ( (frontVert != endVert) || (vertBackLogIdx > 1) ) {
 			if (vertBackLogIdx > 0) {
 				// vertBackLog is not empty
-				if ( PNLTRI.Math.ptsCrossProd( frontVert, vertBackLog[vertBackLogIdx-1], vertBackLog[vertBackLogIdx] ) > 0 ) {		// TODO !!
+				var angle = PNLTRI.Math.ptsCrossProd( frontVert, vertBackLog[vertBackLogIdx-1], vertBackLog[vertBackLogIdx] );		// TODO !!
+				if ( Math.abs(angle) <= PNLTRI.Math.EPSILON_P ) {
+					// co-linear
+					if ( PNLTRI.Math.compare_pts_yx( frontVert, vertBackLog[vertBackLogIdx] ) != PNLTRI.Math.compare_pts_yx( vertBackLog[vertBackLogIdx], vertBackLog[vertBackLogIdx-1] ) ) {
+//						console.log("triangulate_monotone_polygon: colinear", frontVert.x - vertBackLog[vertBackLogIdx].x, frontVert.y - vertBackLog[vertBackLogIdx].y,
+//													vertBackLog[vertBackLogIdx].x - vertBackLog[vertBackLogIdx-1].x, vertBackLog[vertBackLogIdx].y - vertBackLog[vertBackLogIdx-1].y,
+//													frontVert, vertBackLog[vertBackLogIdx], vertBackLog[vertBackLogIdx-1] );
+						angle = 1;		// co-linear-reversal => create triangle
+					}
+				}
+				if ( angle > 0 ) {
 					// convex corner: cut if off
 					this.polyData.addTriangle( vertBackLog[vertBackLogIdx-1], vertBackLog[vertBackLogIdx], frontVert );
 					vertBackLogIdx--;
diff --git a/test/MonoSplitterTest.js b/test/MonoSplitterTest.js
index 3d91063..2124d20 100644
--- a/test/MonoSplitterTest.js
+++ b/test/MonoSplitterTest.js
@@ -18,7 +18,7 @@ PNLTRI.MonoSplitter.prototype.alyTrap_check = function ( inTrap, inFromUp, inFro
 	ok( inTrap.monoDone, inTestName );
 };
 PNLTRI.MonoSplitter.prototype.mockSetup = function () {
-	
+
 	function mock_doSplit_check( inChain, inVertLow, inVertHigh, inLow2High ) {
 		if ( mockDoChecks )		return	mock_check( [ inChain, inVertLow, inVertHigh, inLow2High ] );
 		return	null;
@@ -30,12 +30,12 @@ PNLTRI.MonoSplitter.prototype.mockSetup = function () {
 	this.doSplit = mock_doSplit_check;
 	mock_check_off();
 };
-	
+
 function test_MonoSplitter() {
-	
+
 	/* 4 Cases for "high points", mirror them for "low points"
-	
-	/*   L 
+
+	/*   L
 	 *							  /
 	 *   -----*------------------/
 	 *		   \      Trap		/
@@ -69,16 +69,16 @@ function test_MonoSplitter() {
 	 *	TR_BR, TR_BLR: no diag
 	 *
 	 *	TM_BL, TM_BM, TM_BR, TM_BLR: diag
-	 *  
+	 *
 	 *	TLR_BL, TLR_BR: no diag
 	 *	TLR_BM: diag
 	 *
 	 *	TLR_BLR: not possible
 	 */
-	
+
 	/**************************************************************************/
-	 
-	/*    
+
+	/*
 	 *						  /
 	 *	------*--------------/
 	 *		   \      		/
@@ -121,8 +121,8 @@ function test_MonoSplitter() {
 		mock_set_expected( [ [ myTrap.uR, false, false, 0 ] ] );
 		myMono.alyTrap_check( myTrap, false, false, "TL_BL: from dR, no diag" );
 	}
-	
-	/*    
+
+	/*
 	 *	 \
 	 *	  \--------------*----
 	 *	   \      		/
@@ -162,8 +162,8 @@ function test_MonoSplitter() {
 		myMono.alyTrap_check( myTrap, false, true, "TR_BR: from dL, no diag" );
 	}
 
-	/*    
-	 *				 /	
+	/*
+	 *				 /
 	 *	------*-----/
 	 *		   \   /
 	 *			\ /
@@ -200,7 +200,7 @@ function test_MonoSplitter() {
 		myMono.alyTrap_check( myTrap, true, false, "TL_BLR: from uR, no diag" );
 	}
 
-	/*    
+	/*
 	 *		 \
 	 *	      \-----*-----
 	 *		   \   /
@@ -237,8 +237,8 @@ function test_MonoSplitter() {
 		mock_set_expected( [ [ myTrap.uL, false, true, 0 ] ] );
 		myMono.alyTrap_check( myTrap, true, true, "TR_BLR: from uL, no diag" );
 	}
-	
-	/*    
+
+	/*
 	 *
 	 *	--------*------
 	 *		   / \
@@ -275,8 +275,8 @@ function test_MonoSplitter() {
 		mock_set_expected( [ [ myTrap.dR, true, false, 0 ] ] );
 		myMono.alyTrap_check( myTrap, false, false, "TLR_BL: from dR, no diag" );
 	}
-	
-	/*    
+
+	/*
 	 *
 	 *	--------*------
 	 *		   / \
@@ -313,8 +313,8 @@ function test_MonoSplitter() {
 		mock_set_expected( [ [ myTrap.dL, true, true, 0 ] ] );
 		myMono.alyTrap_check( myTrap, false, true, "TLR_BR: from dL, no diag" );
 	}
-	
-	/*    
+
+	/*
 	 *							  /
 	 *	------*------------------/
 	 *		   \      			/
@@ -357,8 +357,8 @@ function test_MonoSplitter() {
 								[ myTrap.uR, false, false, 7 ] ], [	7 ] );
 		myMono.alyTrap_check( myTrap, false, true, "TL_BR: from dL, diag: vLow(right)->vHigh(left)" );
 	}
-	
-	/*    
+
+	/*
 	 *	 \
 	 *	  \------------------*-----
 	 *	   \      			/
@@ -401,8 +401,8 @@ function test_MonoSplitter() {
 								[ myTrap.uL, false, true, 7 ] ], [	7 ] );
 		myMono.alyTrap_check( myTrap, false, false, "TR_BL: from dR, diag: vHigh(right)->vLow(left)" );
 	}
-	
-	/*    
+
+	/*
 	 *		 					  /
 	 *	------*------------------/
 	 *		   \      			/
@@ -455,8 +455,8 @@ function test_MonoSplitter() {
 								[ myTrap.uR, false, false, 7 ] ], [	7 ] );
 		myMono.alyTrap_check( myTrap, false, true, "TL_BM: from dL, diag: vLow(middle)->vHigh(left)" );
 	}
-	
-	/*    
+
+	/*
 	 *	 \
 	 *	  \------------------*----
 	 *	   \      			/
@@ -509,8 +509,8 @@ function test_MonoSplitter() {
 								[ myTrap.uL, false, true, 7 ] ], [	7 ] );
 		myMono.alyTrap_check( myTrap, false, false, "TR_BM: from dR, diag: vHigh(right)->vLow(middle)" );
 	}
-	
-	/*    
+
+	/*
 	 *	 \		  \ /		  /
 	 *	  \--------*---------/
 	 *	   \      			/
@@ -563,8 +563,8 @@ function test_MonoSplitter() {
 								[ myTrap.uL, false, true, 7 ] ], [	7 ] );
 		myMono.alyTrap_check( myTrap, false, false, "TM_BL: from dR, diag: vHigh(middle)->vLow(left)" );
 	}
-	
-	/*    
+
+	/*
 	 *	 \		  \ /		  /
 	 *	  \--------*---------/
 	 *	   \      			/
@@ -617,8 +617,8 @@ function test_MonoSplitter() {
 								[ myTrap.uR, false, false, 7 ] ], [	7 ] );
 		myMono.alyTrap_check( myTrap, false, true, "TM_BR: from dL, diag: vLow(right)->vHigh(middle)" );
 	}
-	
-	/*    
+
+	/*
 	 *	 \		  \ /		  /
 	 *	  \--------*---------/
 	 *	   \      			/
@@ -685,7 +685,7 @@ function test_MonoSplitter() {
 		myMono.alyTrap_check( myTrap, false, false, "TM_BM: from dR, diag: vHigh(middle)->vLow(middle)" );
 	}
 
-	/*    
+	/*
 	 *	------*-------
 	 *		 / \
 	 *		/	\
@@ -733,8 +733,8 @@ function test_MonoSplitter() {
 								[ myTrap.dL, true, true, 7 ] ], [	7 ] );
 		myMono.alyTrap_check( myTrap, false, false, "TLR_BM: from dR, diag: vHigh->vLow(middle)" );
 	}
-	
-	/*    
+
+	/*
 	 *	   \  \ /  /
 	 *		\--*--/
 	 *		 \	 /
@@ -782,10 +782,10 @@ function test_MonoSplitter() {
 								[ myTrap.uL, false, true, 7 ] ], [	7 ] );
 		myMono.alyTrap_check( myTrap, true, false, "TM_BLR: from uR, diag: vHigh(middle)->vLow" );
 	}
-	
+
 	/**************************************************************************/
 
-	/*    
+	/*
 	 *	 \
 	 *	  \------------------*----
 	 *	   \      			/
@@ -845,7 +845,7 @@ function test_MonoSplitter() {
 								[ myTrap.uL, false, true, 7 ] ], [	7 ] );
 		myMono.alyTrap_check( myTrap, false, false, "TR_BM__c_CCW_h_CW: from dR, diag: vHigh(right)->vLow(middle)" );
 	}
-	
+
 	 function test_TR_BM__c_CW_h_CCW() {
 		var myPolygonData = new PNLTRI.PolygonData( [
 				[	// contour: CW
@@ -898,7 +898,7 @@ function test_MonoSplitter() {
 								[ myTrap.uL, false, true, 7 ] ], [	7 ] );
 		myMono.alyTrap_check( myTrap, false, false, "TR_BM__c_CW_h_CCW: from dR, diag: vHigh(right)->vLow(middle)" );
 	}
-	
+
 	/**************************************************************************/
 
 	var	testData = new PolygonTestdata();
@@ -932,7 +932,7 @@ function test_MonoSplitter() {
 		}
 	}
 
-		
+
 	test( "Polygon Monotone Splitter", function() {
 		// contour: CCW; no hole
 		//	no diagonal
diff --git a/test/MonoTriangulatorTest.js b/test/MonoTriangulatorTest.js
index 0e48826..7f02624 100644
--- a/test/MonoTriangulatorTest.js
+++ b/test/MonoTriangulatorTest.js
@@ -6,11 +6,11 @@ function test_MonoTriangulator() {
 
 	var	testData = new PolygonTestdata();
 
-	
+
 	/***************************************************************************
 	 *	Tests
 	 **************************************************************************/
-	
+
 	function test_triangulate_all_polygons( inDataName, inDrawScale ) {
 		var baseData = testData.get_polygon_with_holes( inDataName );
 		var expectedTriangList	= testData.get_triangles( inDataName );
@@ -56,7 +56,7 @@ function test_MonoTriangulator() {
 	function test_triangulate_monotone_polygon() {
 
 		var myPolygonData, myMonoTriang, myMonoChain, triangList;
-		
+
 		// Helper function
 		function initSeglistMonoChain( inPtList ) {
 			myPolygonData = new PNLTRI.PolygonData( [ inPtList ] );
@@ -140,11 +140,45 @@ function test_MonoTriangulator() {
 		equal( triangList.length, 0, "No Triangle from 1 vertex" );
 	}
 
-	
+	function test_triangulate_monotone_polygon_colinear() {
+
+		var myPolygonData, myMonoTriang, myMonoChain, triangList;
+
+		// Helper function
+		function initSeglistMonoChain( inPtList ) {
+			myPolygonData = new PNLTRI.PolygonData( [ inPtList ] );
+			myPolygonData.initMonoChains();
+			myMonoTriang = new PNLTRI.MonoTriangulator( myPolygonData );
+			//
+			myMonoChain = myPolygonData.getSegments();
+			triangList = myPolygonData.getTriangleList();		// unsorted results !!
+		}
+
+		var myVertices;
+		//
+		// co-linear reversal
+		myVertices = [ { x:30,y:30 }, { x:10,y:10 }, { x:25,y:14 }, { x:20,y:20 } ];
+		initSeglistMonoChain( myVertices );
+		myMonoTriang.triangulate_monotone_polygon( myMonoChain[0] );
+		equal( triangList.length, 2, "co-linear reversal #1: number" );
+		deepEqual(	triangList, [ [ 1, 2, 3 ], [ 1, 3, 0 ] ], "co-linear reversal #1: Triangles" );
+//		drawPolygonLayers( { "poly": [ myVertices ], "triang": myPolygonData.triangles_2_polygons() } );
+		//
+		// co-linear reversal
+		myVertices = [ { x:40,y:40 }, { x:29,y:37 }, { x:30,y:30 }, { x:20,y:20 } ];
+		initSeglistMonoChain( myVertices );
+		myMonoTriang.triangulate_monotone_polygon( myMonoChain[0] );
+		equal( triangList.length, 2, "co-linear reversal #2: number" );			// TODO: Error
+		deepEqual(	triangList, [ [ 2, 3, 0 ], [ 1, 2, 0 ] ], "co-linear reversal #2: Triangles" );
+//		drawPolygonLayers( { "poly": [ myVertices ], "triang": myPolygonData.triangles_2_polygons() } );
+	}
+
+
 	test( "Triangulator for uni-Y-monotone Polygons", function() {
 		test_triangulate_all_polygons( "square_3triangholes", 5 );
 		test_triangulate_all_polygons( "pt_3_diag_max", 4 );
 		test_triangulate_monotone_polygon();
+		test_triangulate_monotone_polygon_colinear();
 	});
 }
 
diff --git a/test/TrapezoiderTest.js b/test/TrapezoiderTest.js
index b82a8e9..75b3418 100644
--- a/test/TrapezoiderTest.js
+++ b/test/TrapezoiderTest.js
@@ -800,6 +800,8 @@ function test_QueryStructure() {
 		ok( ( myQs.ptNode( { vFrom: secondPoint, vTo: { x: 4, y: 1 }, rootFrom: qsLR }, true ) == qs_tr3 ), "ptNode B: X-Node(qsLR), =vTo, horiz -> qs_tr3" );
 	}
 
+	/**************************************************************************/
+
 	//
 	//	Cases of endpoints touching other segments
 	//	The touched segment is already inserted, now the endpoint of another segment,
@@ -807,12 +809,6 @@ function test_QueryStructure() {
 	//	This is the opposite case to "test_add_segment_touching_N"
 	//
 
-		// TODO: test all mirrored cases !!!
-		//	if going back co-linear direction
-		//	should be checked with the preceding segment
-		// ATTENTION: this needs to be looped since the
-		//	preceeding segment can also be co-linear ...
-
 	function test_ptNode_touching() {
 
 		var testPolygon = [ { x: 10, y: 30 }, { x: 20, y: 10 }, { x: 30, y: 40 } ];
@@ -846,7 +842,7 @@ function test_QueryStructure() {
 		ok( ( testSegment.rootTo == qs_tr3 ), "ptNode_touching: up reverse, right -> qs_tr3" );
 	}
 
-	function test_ptNode_colinear_1() {		// TODO: with vertical lines
+	function test_ptNode_colinear_inside() {
 
 		// fully left side, CCW
 		//
@@ -874,16 +870,16 @@ function test_QueryStructure() {
 
 		//	prev(co-linear) segment
 		myQs.segNodes( segListArray[3] );
-		ok( ( segListArray[3].rootFrom == qs_tr1 ), "ptNode_colinear#1 A: prev, left -> qs_tr1" );
-		ok( ( segListArray[3].rootTo == qs_tr1 ), "ptNode_colinear#1 A: prev reverse, left -> qs_tr1" );
+		ok( ( segListArray[3].rootFrom == qs_tr1 ), "ptNode_colinear_inside A: prev, left -> qs_tr1" );
+		ok( ( segListArray[3].rootTo == qs_tr1 ), "ptNode_colinear_inside A: prev reverse, left -> qs_tr1" );
 		//	co-linear segment
 		myQs.segNodes( segListArray[4] );
-		ok( ( segListArray[4].rootFrom == qs_tr1 ), "ptNode_colinear#1 A: co-lin, left -> qs_tr1" );
-		ok( ( segListArray[4].rootTo == qs_tr1 ), "ptNode_colinear#1 A: co-lin reverse, left -> qs_tr1" );
+		ok( ( segListArray[4].rootFrom == qs_tr1 ), "ptNode_colinear_inside A: co-lin, left -> qs_tr1" );
+		ok( ( segListArray[4].rootTo == qs_tr1 ), "ptNode_colinear_inside A: co-lin reverse, left -> qs_tr1" );
 		//	next(co-linear) segment
 		myQs.segNodes( segListArray[5] );
-		ok( ( segListArray[5].rootFrom == qs_tr1 ), "ptNode_colinear#1 A: next, left -> qs_tr1" );
-		ok( ( segListArray[5].rootTo == qs_tr1 ), "ptNode_colinear#1 A: next reverse, left -> qs_tr1" );
+		ok( ( segListArray[5].rootFrom == qs_tr1 ), "ptNode_colinear_inside A: next, left -> qs_tr1" );
+		ok( ( segListArray[5].rootTo == qs_tr1 ), "ptNode_colinear_inside A: next reverse, left -> qs_tr1" );
 
 		//
 		// horizontal lines
@@ -903,25 +899,60 @@ function test_QueryStructure() {
 
 		//	prev(prev(co-linear)) segment
 		myQs.segNodes( segListArray[2] );
-		ok( ( segListArray[2].rootFrom == qs_tr0 ), "ptNode_colinear#1 B: prev-prev, left -> qs_tr0" );
-		ok( ( segListArray[2].rootTo == qs_tr0 ), "ptNode_colinear#1 B: prev-prev reverse, left -> qs_tr0" );
+		ok( ( segListArray[2].rootFrom == qs_tr0 ), "ptNode_colinear_inside B: prev-prev, left -> qs_tr0" );
+		ok( ( segListArray[2].rootTo == qs_tr0 ), "ptNode_colinear_inside B: prev-prev reverse, left -> qs_tr0" );
 		//	prev(co-linear) segment
 		myQs.segNodes( segListArray[3] );
-		ok( ( segListArray[3].rootFrom == qs_tr0 ), "ptNode_colinear#1 B: prev, left -> qs_tr1" );
-		ok( ( segListArray[3].rootTo == qs_tr1 ), "ptNode_colinear#1 B: prev reverse, left -> qs_tr1" );
+		ok( ( segListArray[3].rootFrom == qs_tr0 ), "ptNode_colinear_inside B: prev, left -> qs_tr0" );
+		ok( ( segListArray[3].rootTo == qs_tr1 ), "ptNode_colinear_inside B: prev reverse, left -> qs_tr1" );
 		//	co-linear segment
 		myQs.segNodes( segListArray[4] );
-		ok( ( segListArray[4].rootFrom == qs_tr1 ), "ptNode_colinear#1 B: co-lin, left -> qs_tr1" );
-		ok( ( segListArray[4].rootTo == qs_tr1 ), "ptNode_colinear#1 B: co-lin reverse, left -> qs_tr1" );
+		ok( ( segListArray[4].rootFrom == qs_tr1 ), "ptNode_colinear_inside B: co-lin, left -> qs_tr1" );
+		ok( ( segListArray[4].rootTo == qs_tr1 ), "ptNode_colinear_inside B: co-lin reverse, left -> qs_tr1" );
 		//	next(co-linear) segment
 		myQs.segNodes( segListArray[5] );
-		ok( ( segListArray[5].rootFrom == qs_tr1 ), "ptNode_colinear#1 B: next, left -> qs_tr1" );
-		ok( ( segListArray[5].rootTo == qs_tr0 ), "ptNode_colinear#1 B: next reverse, left -> qs_tr1" );
+		ok( ( segListArray[5].rootFrom == qs_tr1 ), "ptNode_colinear_inside B: next, left -> qs_tr1" );
+		ok( ( segListArray[5].rootTo == qs_tr0 ), "ptNode_colinear_inside B: next reverse, left -> qs_tr0" );
 		//	next(next(co-linear)) segment
 		myQs.segNodes( segListArray[0] );
-		ok( ( segListArray[0].rootFrom == qs_tr0 ), "ptNode_colinear#1 B: next-next, left -> qs_tr0" );
-		ok( ( segListArray[0].rootTo == qs_tr1 ), "ptNode_colinear#1 B: next-next reverse, left -> qs_tr1" );
+		ok( ( segListArray[0].rootFrom == qs_tr0 ), "ptNode_colinear_inside B: next-next, left -> qs_tr0" );
+		ok( ( segListArray[0].rootTo == qs_tr1 ), "ptNode_colinear_inside B: next-next reverse, left -> qs_tr1" );
 
+		//
+		// vertical lines
+		var testPolygon = [ { x: 15, y: 15 }, { x: 30, y: 10 }, { x: 30, y: 40 },
+							{ x: 25, y: 35 }, { x: 30, y: 30 }, { x: 30, y: 20 } ];
+		var myPolygonData = new PNLTRI.PolygonData( [ testPolygon ] );
+		var segListArray = myPolygonData.getSegments();
+
+		var myQs = new PNLTRI.QueryStructure( myPolygonData );
+		var myQsRoot = myQs.getRoot();
+
+		myQs.add_segment( segListArray[1] );		// touch line, 2nd touch line: segListArray[4]
+		var qs_tr1 = myQs.getTrapByIdx(1).sink;
+//		showDataStructure( myQsRoot );
+//		drawTrapezoids( myQsRoot, false );
+
+		//	prev(prev(co-linear)) segment
+		myQs.segNodes( segListArray[2] );
+		ok( ( segListArray[2].rootFrom == qs_tr1 ), "ptNode_colinear_inside C: prev-prev, left -> qs_tr1" );
+		ok( ( segListArray[2].rootTo == qs_tr1 ), "ptNode_colinear_inside C: prev-prev reverse, left -> qs_tr1" );
+		//	prev(co-linear) segment
+		myQs.segNodes( segListArray[3] );
+		ok( ( segListArray[3].rootFrom == qs_tr1 ), "ptNode_colinear_inside C: prev, left -> qs_tr1" );
+		ok( ( segListArray[3].rootTo == qs_tr1 ), "ptNode_colinear_inside C: prev reverse, left -> qs_tr1" );
+		//	co-linear segment
+		myQs.segNodes( segListArray[4] );
+		ok( ( segListArray[4].rootFrom == qs_tr1 ), "ptNode_colinear_inside C: co-lin, left -> qs_tr1" );
+		ok( ( segListArray[4].rootTo == qs_tr1 ), "ptNode_colinear_inside C: co-lin reverse, left -> qs_tr1" );
+		//	next(co-linear) segment
+		myQs.segNodes( segListArray[5] );
+		ok( ( segListArray[5].rootFrom == qs_tr1 ), "ptNode_colinear_inside C: next, left -> qs_tr1" );
+		ok( ( segListArray[5].rootTo == qs_tr1 ), "ptNode_colinear_inside C: next reverse, left -> qs_tr1" );
+		//	next(next(co-linear)) segment
+		myQs.segNodes( segListArray[0] );
+		ok( ( segListArray[0].rootFrom == qs_tr1 ), "ptNode_colinear_inside C: next-next, left -> qs_tr1" );
+		ok( ( segListArray[0].rootTo == qs_tr1 ), "ptNode_colinear_inside C: next-next reverse, left -> qs_tr1" );
 
 		// fully right side, CW
 		//
@@ -949,16 +980,16 @@ function test_QueryStructure() {
 
 		//	prev(co-linear) segment
 		myQs.segNodes( segListArray[3] );
-		ok( ( segListArray[3].rootFrom == qs_tr3 ), "ptNode_colinear#1 C: prev, right -> qs_tr3" );
-		ok( ( segListArray[3].rootTo == qs_tr3 ), "ptNode_colinear#1 C: prev reverse, right -> qs_tr3" );
+		ok( ( segListArray[3].rootFrom == qs_tr3 ), "ptNode_colinear_inside D: prev, right -> qs_tr3" );
+		ok( ( segListArray[3].rootTo == qs_tr3 ), "ptNode_colinear_inside D: prev reverse, right -> qs_tr3" );
 		//	co-linear segment
 		myQs.segNodes( segListArray[4] );
-		ok( ( segListArray[4].rootFrom == qs_tr3 ), "ptNode_colinear#1 C: co-lin, right -> qs_tr3" );
-		ok( ( segListArray[4].rootTo == qs_tr3 ), "ptNode_colinear#1 C: co-lin reverse, right -> qs_tr3" );
+		ok( ( segListArray[4].rootFrom == qs_tr3 ), "ptNode_colinear_inside D: co-lin, right -> qs_tr3" );
+		ok( ( segListArray[4].rootTo == qs_tr3 ), "ptNode_colinear_inside D: co-lin reverse, right -> qs_tr3" );
 		//	next(co-linear) segment
 		myQs.segNodes( segListArray[5] );
-		ok( ( segListArray[5].rootFrom == qs_tr3 ), "ptNode_colinear#1 C: next, right -> qs_tr3" );
-		ok( ( segListArray[5].rootTo == qs_tr3 ), "ptNode_colinear#1 C: next reverse, right -> qs_tr3" );
+		ok( ( segListArray[5].rootFrom == qs_tr3 ), "ptNode_colinear_inside D: next, right -> qs_tr3" );
+		ok( ( segListArray[5].rootTo == qs_tr3 ), "ptNode_colinear_inside D: next reverse, right -> qs_tr3" );
 
 		//
 		// horizontal lines
@@ -978,27 +1009,63 @@ function test_QueryStructure() {
 
 		//	prev(prev(co-linear)) segment
 		myQs.segNodes( segListArray[2] );
-		ok( ( segListArray[2].rootFrom == qs_tr3 ), "ptNode_colinear#1 D: prev-prev, right -> qs_tr3" );
-		ok( ( segListArray[2].rootTo == qs_tr2 ), "ptNode_colinear#1 D: prev-prev reverse, right -> qs_tr2" );
+		ok( ( segListArray[2].rootFrom == qs_tr3 ), "ptNode_colinear_inside E: prev-prev, right -> qs_tr3" );
+		ok( ( segListArray[2].rootTo == qs_tr2 ), "ptNode_colinear_inside E: prev-prev reverse, right -> qs_tr2" );
+		//	prev(co-linear) segment
+		myQs.segNodes( segListArray[3] );
+		ok( ( segListArray[3].rootFrom == qs_tr2 ), "ptNode_colinear_inside E: prev, right -> qs_tr2" );
+		ok( ( segListArray[3].rootTo == qs_tr3 ), "ptNode_colinear_inside E: prev reverse, right -> qs_tr3" );
+		//	co-linear segment
+		myQs.segNodes( segListArray[4] );
+		ok( ( segListArray[4].rootFrom == qs_tr3 ), "ptNode_colinear_inside E: co-lin, right -> qs_tr3" );
+		ok( ( segListArray[4].rootTo == qs_tr3 ), "ptNode_colinear_inside E: co-lin reverse, right -> qs_tr3" );
+		//	next(co-linear) segment
+		myQs.segNodes( segListArray[5] );
+		ok( ( segListArray[5].rootFrom == qs_tr3 ), "ptNode_colinear_inside E: next, right -> qs_tr3" );
+		ok( ( segListArray[5].rootTo == qs_tr2 ), "ptNode_colinear_inside E: next reverse, right -> qs_tr2" );
+		//	next(next(co-linear)) segment
+		myQs.segNodes( segListArray[0] );
+		ok( ( segListArray[0].rootFrom == qs_tr2 ), "ptNode_colinear_inside E: next-next, right -> qs_tr2" );
+		ok( ( segListArray[0].rootTo == qs_tr2 ), "ptNode_colinear_inside E: next-next reverse, right -> qs_tr2" );
+
+		//
+		// vertical lines
+		var testPolygon = [ { x: 45, y: 15 }, { x: 30, y: 10 }, { x: 30, y: 40 },
+							{ x: 35, y: 35 }, { x: 30, y: 30 }, { x: 30, y: 20 } ];
+		var myPolygonData = new PNLTRI.PolygonData( [ testPolygon ] );
+		var segListArray = myPolygonData.getSegments();
+
+		var myQs = new PNLTRI.QueryStructure( myPolygonData );
+		var myQsRoot = myQs.getRoot();
+
+		myQs.add_segment( segListArray[1] );		// touch line, 2nd touch line: segListArray[4]
+		var qs_tr3 = myQs.getTrapByIdx(3).sink;
+//		showDataStructure( myQsRoot );
+//		drawTrapezoids( myQsRoot, false );
+
+		//	prev(prev(co-linear)) segment
+		myQs.segNodes( segListArray[2] );
+		ok( ( segListArray[2].rootFrom == qs_tr3 ), "ptNode_colinear_inside F: prev-prev, left -> qs_tr3" );
+		ok( ( segListArray[2].rootTo == qs_tr3 ), "ptNode_colinear_inside F: prev-prev reverse, left -> qs_tr3" );
 		//	prev(co-linear) segment
 		myQs.segNodes( segListArray[3] );
-		ok( ( segListArray[3].rootFrom == qs_tr2 ), "ptNode_colinear#1 D: prev, right -> qs_tr2" );
-		ok( ( segListArray[3].rootTo == qs_tr3 ), "ptNode_colinear#1 D: prev reverse, right -> qs_tr3" );
+		ok( ( segListArray[3].rootFrom == qs_tr3 ), "ptNode_colinear_inside F: prev, left -> qs_tr3" );
+		ok( ( segListArray[3].rootTo == qs_tr3 ), "ptNode_colinear_inside F: prev reverse, left -> qs_tr3" );
 		//	co-linear segment
 		myQs.segNodes( segListArray[4] );
-		ok( ( segListArray[4].rootFrom == qs_tr3 ), "ptNode_colinear#1 D: co-lin, right -> qs_tr3" );
-		ok( ( segListArray[4].rootTo == qs_tr3 ), "ptNode_colinear#1 D: co-lin reverse, right -> qs_tr3" );
+		ok( ( segListArray[4].rootFrom == qs_tr3 ), "ptNode_colinear_inside F: co-lin, left -> qs_tr3" );
+		ok( ( segListArray[4].rootTo == qs_tr3 ), "ptNode_colinear_inside F: co-lin reverse, left -> qs_tr3" );
 		//	next(co-linear) segment
 		myQs.segNodes( segListArray[5] );
-		ok( ( segListArray[5].rootFrom == qs_tr3 ), "ptNode_colinear#1 D: next, right -> qs_tr3" );
-		ok( ( segListArray[5].rootTo == qs_tr2 ), "ptNode_colinear#1 D: next reverse, right -> qs_tr2" );
+		ok( ( segListArray[5].rootFrom == qs_tr3 ), "ptNode_colinear_inside F: next, left -> qs_tr3" );
+		ok( ( segListArray[5].rootTo == qs_tr3 ), "ptNode_colinear_inside F: next reverse, left -> qs_tr3" );
 		//	next(next(co-linear)) segment
 		myQs.segNodes( segListArray[0] );
-		ok( ( segListArray[0].rootFrom == qs_tr2 ), "ptNode_colinear#1 D: next-next, right -> qs_tr2" );
-		ok( ( segListArray[0].rootTo == qs_tr2 ), "ptNode_colinear#1 D: next-next reverse, right -> qs_tr2" );
+		ok( ( segListArray[0].rootFrom == qs_tr3 ), "ptNode_colinear_inside F: next-next, left -> qs_tr3" );
+		ok( ( segListArray[0].rootTo == qs_tr3 ), "ptNode_colinear_inside F: next-next reverse, left -> qs_tr3" );
 	}
 
-	function test_ptNode_colinear_2() {
+	function test_ptNode_colinear_overlapping() {
 
 		// overlapping left low, right high, CCW
 		//
@@ -1010,7 +1077,7 @@ function test_QueryStructure() {
 		//         +__     2
 		//
 		var testPolygon = [ { x: 25, y: 14 }, { x: 20, y: 20 }, { x: 40, y: 40 },
-							{ x: 35, y: 37 }, { x: 30, y: 30 }, { x: 10, y: 10 } ];
+							{ x: 29, y: 37 }, { x: 30, y: 30 }, { x: 10, y: 10 } ];
 		var myPolygonData = new PNLTRI.PolygonData( [ testPolygon ] );
 		var segListArray = myPolygonData.getSegments();
 
@@ -1025,16 +1092,16 @@ function test_QueryStructure() {
 
 		//	prev(co-linear) segment
 		myQs.segNodes( segListArray[3] );
-		ok( ( segListArray[3].rootFrom == qs_tr1 ), "ptNode_colinear#2 A: prev, left -> qs_tr1" );
-		ok( ( segListArray[3].rootTo == qs_tr1 ), "ptNode_colinear#2 A: prev reverse, left -> qs_tr1" );
+		ok( ( segListArray[3].rootFrom == qs_tr1 ), "ptNode_colinear_overlapping A: prev, left -> qs_tr1" );
+		ok( ( segListArray[3].rootTo == qs_tr1 ), "ptNode_colinear_overlapping A: prev reverse, left -> qs_tr1" );
 		//	co-linear segment
 		myQs.segNodes( segListArray[4] );
-		ok( ( segListArray[4].rootFrom == qs_tr1 ), "ptNode_colinear#2 A: co-lin, left -> qs_tr1" );
-		ok( ( segListArray[4].rootTo == qs_tr2 ), "ptNode_colinear#2 A: co-lin reverse, left -> qs_tr2" );
+		ok( ( segListArray[4].rootFrom == qs_tr1 ), "ptNode_colinear_overlapping A: co-lin, left -> qs_tr1" );
+		ok( ( segListArray[4].rootTo == qs_tr2 ), "ptNode_colinear_overlapping A: co-lin reverse, left -> qs_tr2" );
 		//	next(co-linear) segment
 		myQs.segNodes( segListArray[5] );
-		ok( ( segListArray[5].rootFrom == qs_tr2 ), "ptNode_colinear#2 A: next, left -> qs_tr2" );
-		ok( ( segListArray[5].rootTo == qs_tr2 ), "ptNode_colinear#2 A: next reverse, left -> qs_tr2" );
+		ok( ( segListArray[5].rootFrom == qs_tr2 ), "ptNode_colinear_overlapping A: next, left -> qs_tr2" );
+		ok( ( segListArray[5].rootTo == qs_tr2 ), "ptNode_colinear_overlapping A: next reverse, left -> qs_tr2" );
 
 		//
 		// other sequence
@@ -1052,16 +1119,16 @@ function test_QueryStructure() {
 
 		//	prev(co-linear) segment
 		myQs.segNodes( segListArray[0] );
-		ok( ( segListArray[0].rootFrom == qs_tr3 ), "ptNode_colinear#2 B: prev, right -> qs_tr3" );
-		ok( ( segListArray[0].rootTo == qs_tr3 ), "ptNode_colinear#2 B: prev reverse, right -> qs_tr3" );
+		ok( ( segListArray[0].rootFrom == qs_tr3 ), "ptNode_colinear_overlapping B: prev, right -> qs_tr3" );
+		ok( ( segListArray[0].rootTo == qs_tr3 ), "ptNode_colinear_overlapping B: prev reverse, right -> qs_tr3" );
 		//	co-linear segment
 		myQs.segNodes( segListArray[1] );
-		ok( ( segListArray[1].rootFrom == qs_tr3 ), "ptNode_colinear#2 B: co-lin, right -> qs_tr3" );
-		ok( ( segListArray[1].rootTo == qs_tr0 ), "ptNode_colinear#1 B: co-lin reverse, right -> qs_tr0" );
+		ok( ( segListArray[1].rootFrom == qs_tr3 ), "ptNode_colinear_overlapping B: co-lin, right -> qs_tr3" );
+		ok( ( segListArray[1].rootTo == qs_tr0 ), "ptNode_colinear_overlapping B: co-lin reverse, right -> qs_tr0" );
 		//	next(co-linear) segment
 		myQs.segNodes( segListArray[2] );
-		ok( ( segListArray[2].rootFrom == qs_tr0 ), "ptNode_colinear#2 B: next, right -> qs_tr0" );
-		ok( ( segListArray[2].rootTo == qs_tr0 ), "ptNode_colinear#1 B: next reverse, right -> qs_tr0" );
+		ok( ( segListArray[2].rootFrom == qs_tr0 ), "ptNode_colinear_overlapping B: next, right -> qs_tr0" );
+		ok( ( segListArray[2].rootTo == qs_tr0 ), "ptNode_colinear_overlapping B: next reverse, right -> qs_tr0" );
 
 
 		// overlapping right low, left high, CW
@@ -1089,16 +1156,16 @@ function test_QueryStructure() {
 
 		//	prev(co-linear) segment
 		myQs.segNodes( segListArray[3] );
-		ok( ( segListArray[3].rootFrom == qs_tr3 ), "ptNode_colinear#2 C: prev, right -> qs_tr3" );
-		ok( ( segListArray[3].rootTo == qs_tr3 ), "ptNode_colinear#2 C: prev reverse, right -> qs_tr3" );
+		ok( ( segListArray[3].rootFrom == qs_tr3 ), "ptNode_colinear_overlapping C: prev, right -> qs_tr3" );
+		ok( ( segListArray[3].rootTo == qs_tr3 ), "ptNode_colinear_overlapping C: prev reverse, right -> qs_tr3" );
 		//	co-linear segment
 		myQs.segNodes( segListArray[4] );
-		ok( ( segListArray[4].rootFrom == qs_tr3 ), "ptNode_colinear#2 C: co-lin, right -> qs_tr3" );
-		ok( ( segListArray[4].rootTo == qs_tr2 ), "ptNode_colinear#1 C: co-lin reverse, right -> qs_tr2" );
+		ok( ( segListArray[4].rootFrom == qs_tr3 ), "ptNode_colinear_overlapping C: co-lin, right -> qs_tr3" );
+		ok( ( segListArray[4].rootTo == qs_tr2 ), "ptNode_colinear_overlapping C: co-lin reverse, right -> qs_tr2" );
 		//	next(co-linear) segment
 		myQs.segNodes( segListArray[5] );
-		ok( ( segListArray[5].rootFrom == qs_tr2 ), "ptNode_colinear#2 C: next, right -> qs_tr2" );
-		ok( ( segListArray[5].rootTo == qs_tr2 ), "ptNode_colinear#1 C: next reverse, right -> qs_tr2" );
+		ok( ( segListArray[5].rootFrom == qs_tr2 ), "ptNode_colinear_overlapping C: next, right -> qs_tr2" );
+		ok( ( segListArray[5].rootTo == qs_tr2 ), "ptNode_colinear_overlapping C: next reverse, right -> qs_tr2" );
 
 		//
 		// other sequence
@@ -1116,18 +1183,149 @@ function test_QueryStructure() {
 
 		//	prev(co-linear) segment
 		myQs.segNodes( segListArray[0] );
-		ok( ( segListArray[0].rootFrom == qs_tr1 ), "ptNode_colinear#2 D: prev, left -> qs_tr1" );
-		ok( ( segListArray[0].rootTo == qs_tr1 ), "ptNode_colinear#2 D: prev reverse, left -> qs_tr1" );
+		ok( ( segListArray[0].rootFrom == qs_tr1 ), "ptNode_colinear_overlapping D: prev, left -> qs_tr1" );
+		ok( ( segListArray[0].rootTo == qs_tr1 ), "ptNode_colinear_overlapping D: prev reverse, left -> qs_tr1" );
 		//	co-linear segment
 		myQs.segNodes( segListArray[1] );
-		ok( ( segListArray[1].rootFrom == qs_tr1 ), "ptNode_colinear#2 D: co-lin, left -> qs_tr1" );
-		ok( ( segListArray[1].rootTo == qs_tr0 ), "ptNode_colinear#1 D: co-lin reverse, left -> qs_tr0" );
+		ok( ( segListArray[1].rootFrom == qs_tr1 ), "ptNode_colinear_overlapping D: co-lin, left -> qs_tr1" );
+		ok( ( segListArray[1].rootTo == qs_tr0 ), "ptNode_colinear_overlapping D: co-lin reverse, left -> qs_tr0" );
 		//	next(co-linear) segment
 		myQs.segNodes( segListArray[2] );
-		ok( ( segListArray[2].rootFrom == qs_tr0 ), "ptNode_colinear#2 D: next, left -> qs_tr0" );
-		ok( ( segListArray[2].rootTo == qs_tr0 ), "ptNode_colinear#1 D: next reverse, left -> qs_tr0" );
+		ok( ( segListArray[2].rootFrom == qs_tr0 ), "ptNode_colinear_overlapping D: next, left -> qs_tr0" );
+		ok( ( segListArray[2].rootTo == qs_tr0 ), "ptNode_colinear_overlapping D: next reverse, left -> qs_tr0" );
 	}
 
+	function test_ptNode_colinear_reversal() {
+
+		// overlapping left low, right high, CCW
+		//
+		//					0
+		//   -----------*---------------	myQsRoot
+		//  		 + /
+		//  	1	 +/        3
+		//  		+/
+		//   -------*-------------------	qsL
+		//			     2
+		//
+		var testPolygon = [ { x: 20, y: 20 }, { x: 40, y: 40 },
+							{ x: 29, y: 37 }, { x: 30, y: 30 } ];
+		var myPolygonData = new PNLTRI.PolygonData( [ testPolygon ] );
+		var segListArray = myPolygonData.getSegments();
+
+		var myQs = new PNLTRI.QueryStructure( myPolygonData );
+		var myQsRoot = myQs.getRoot();
+
+		myQs.add_segment( segListArray[0] );		// touch line, 2nd touch line: segListArray[3]
+		var qs_tr1 = myQs.getTrapByIdx(1).sink;
+		var qs_tr3 = myQs.getTrapByIdx(3).sink;
+//		showDataStructure( myQsRoot );
+//		drawTrapezoids( myQsRoot, false );
+
+		//	prev(co-linear) segment
+		myQs.segNodes( segListArray[2] );
+		ok( ( segListArray[2].rootFrom == qs_tr1 ), "ptNode_colinear_reversal A: prev, left -> qs_tr1" );
+		ok( ( segListArray[2].rootTo == qs_tr1 ), "ptNode_colinear_reversal A: prev reverse, left -> qs_tr1" );
+		//	co-linear segment
+		myQs.segNodes( segListArray[3] );
+		ok( ( segListArray[3].rootFrom == qs_tr1 ), "ptNode_colinear_reversal A: co-lin, left -> qs_tr1" );
+		ok( ( segListArray[3].rootTo == qs_tr3 ), "ptNode_colinear_reversal A: co-lin reverse, left -> qs_tr1" );		// TODO Error
+		//	next(base) segment
+		myQs.segNodes( segListArray[1] );
+		ok( ( segListArray[1].rootFrom == qs_tr1 ), "ptNode_colinear_reversal A: next(base), left -> qs_tr1" );
+		ok( ( segListArray[1].rootTo == qs_tr1 ), "ptNode_colinear_reversal A: next(base) reverse, left -> qs_tr1" );
+
+		//
+/*		// other sequence
+		var myPolygonData = new PNLTRI.PolygonData( [ testPolygon ] );
+		var segListArray = myPolygonData.getSegments();
+
+		var myQs = new PNLTRI.QueryStructure( myPolygonData );
+		var myQsRoot = myQs.getRoot();
+
+		myQs.add_segment( segListArray[4] );		// touch line, 2nd touch line: segListArray[1]
+		var qs_tr0 = myQs.getTrapByIdx(0).sink;
+		var qs_tr3 = myQs.getTrapByIdx(3).sink;
+//		showDataStructure( myQsRoot );
+//		drawTrapezoids( myQsRoot, false );
+
+		//	prev(co-linear) segment
+		myQs.segNodes( segListArray[0] );
+		ok( ( segListArray[0].rootFrom == qs_tr3 ), "ptNode_colinear_reversal B: prev, right -> qs_tr3" );
+		ok( ( segListArray[0].rootTo == qs_tr3 ), "ptNode_colinear_reversal B: prev reverse, right -> qs_tr3" );
+		//	co-linear segment
+		myQs.segNodes( segListArray[1] );
+		ok( ( segListArray[1].rootFrom == qs_tr3 ), "ptNode_colinear_reversal B: co-lin, right -> qs_tr3" );
+		ok( ( segListArray[1].rootTo == qs_tr0 ), "ptNode_colinear_reversal B: co-lin reverse, right -> qs_tr0" );
+		//	next(co-linear) segment
+		myQs.segNodes( segListArray[2] );
+		ok( ( segListArray[2].rootFrom == qs_tr0 ), "ptNode_colinear_reversal B: next, right -> qs_tr0" );
+		ok( ( segListArray[2].rootTo == qs_tr0 ), "ptNode_colinear_reversal B: next reverse, right -> qs_tr0" );
+
+
+		// overlapping right low, left high, CW
+		//
+		//					0
+		//   ------------*--------------	myQsRoot
+		//  			/
+		//  	1	   /++       3
+		//   ---------*+					qsL
+		// 			  +     2
+		//			+
+		var testPolygon = [ { x:  5, y: 14 }, { x: 20, y: 20 }, { x: 40, y: 40 },
+							{ x: 45, y: 37 }, { x: 30, y: 30 }, { x: 10, y: 10 } ];
+		var myPolygonData = new PNLTRI.PolygonData( [ testPolygon ] );
+		var segListArray = myPolygonData.getSegments();
+
+		var myQs = new PNLTRI.QueryStructure( myPolygonData );
+		var myQsRoot = myQs.getRoot();
+
+		myQs.add_segment( segListArray[1] );		// touch line, 2nd touch line: segListArray[4]
+		var qs_tr2 = myQs.getTrapByIdx(2).sink;
+		var qs_tr3 = myQs.getTrapByIdx(3).sink;
+//		showDataStructure( myQsRoot );
+//		drawTrapezoids( myQsRoot, false );
+
+		//	prev(co-linear) segment
+		myQs.segNodes( segListArray[3] );
+		ok( ( segListArray[3].rootFrom == qs_tr3 ), "ptNode_colinear_reversal C: prev, right -> qs_tr3" );
+		ok( ( segListArray[3].rootTo == qs_tr3 ), "ptNode_colinear_reversal C: prev reverse, right -> qs_tr3" );
+		//	co-linear segment
+		myQs.segNodes( segListArray[4] );
+		ok( ( segListArray[4].rootFrom == qs_tr3 ), "ptNode_colinear_reversal C: co-lin, right -> qs_tr3" );
+		ok( ( segListArray[4].rootTo == qs_tr2 ), "ptNode_colinear_reversal C: co-lin reverse, right -> qs_tr2" );
+		//	next(co-linear) segment
+		myQs.segNodes( segListArray[5] );
+		ok( ( segListArray[5].rootFrom == qs_tr2 ), "ptNode_colinear_reversal C: next, right -> qs_tr2" );
+		ok( ( segListArray[5].rootTo == qs_tr2 ), "ptNode_colinear_reversal C: next reverse, right -> qs_tr2" );
+
+		//
+		// other sequence
+		var myPolygonData = new PNLTRI.PolygonData( [ testPolygon ] );
+		var segListArray = myPolygonData.getSegments();
+
+		var myQs = new PNLTRI.QueryStructure( myPolygonData );
+		var myQsRoot = myQs.getRoot();
+
+		myQs.add_segment( segListArray[4] );		// touch line, 2nd touch line: segListArray[1]
+		var qs_tr0 = myQs.getTrapByIdx(0).sink;
+		var qs_tr1 = myQs.getTrapByIdx(1).sink;
+//		showDataStructure( myQsRoot );
+//		drawTrapezoids( myQsRoot, false );
+
+		//	prev(co-linear) segment
+		myQs.segNodes( segListArray[0] );
+		ok( ( segListArray[0].rootFrom == qs_tr1 ), "ptNode_colinear_reversal D: prev, left -> qs_tr1" );
+		ok( ( segListArray[0].rootTo == qs_tr1 ), "ptNode_colinear_reversal D: prev reverse, left -> qs_tr1" );
+		//	co-linear segment
+		myQs.segNodes( segListArray[1] );
+		ok( ( segListArray[1].rootFrom == qs_tr1 ), "ptNode_colinear_reversal D: co-lin, left -> qs_tr1" );
+		ok( ( segListArray[1].rootTo == qs_tr0 ), "ptNode_colinear_reversal D: co-lin reverse, left -> qs_tr0" );
+		//	next(co-linear) segment
+		myQs.segNodes( segListArray[2] );
+		ok( ( segListArray[2].rootFrom == qs_tr0 ), "ptNode_colinear_reversal D: next, left -> qs_tr0" );
+		ok( ( segListArray[2].rootTo == qs_tr0 ), "ptNode_colinear_reversal D: next reverse, left -> qs_tr0" );
+*/	}
+
 	/**************************************************************************/
 
 
@@ -1838,42 +2036,6 @@ function test_QueryStructure() {
 
 	function test_add_segment_special_5() {
 
-		var myPolygonData = new PNLTRI.PolygonData( [ [
-			{ x: 105, y: 100.4 },
-			{ x: 104, y: 100.39999999999998 },
-			{ x: 103, y: 100.40000000000003 },
-			{ x: 102, y: 100.4 },
-			{ x: 101, y: 100.40000000000002 },
-			] ] );
-		var segListArray = myPolygonData.getSegments();
-//		showDataStructure( myPolygonData.getVertices(), [ 'sprev', 'snext', 'vertTo', 'segOut' ] );
-//		showDataStructure( myPolygonData.getSegments(), [ 'sprev', 'snext', 'mprev', 'mnext' ] );
-		//
-		var myQs = new PNLTRI.QueryStructure( myPolygonData );
-		//
-		myQs.add_segment_consistently( segListArray[0], 'Spec_5 #1' );
-		myQs.add_segment_consistently( segListArray[1], 'Spec_5 #2' );
-		// complex case: EPSILON > rounding effect on coordinates
-		myQs.add_segment_consistently( segListArray[3], 'Spec_5 Main' );
-		equal( myQs.nbTrapezoids(), 9, "Spec_5: Number of Trapezoids" );
-
-		myQs.check_trapezoid_neighbors(  0, null, null, 1, 3, "Spec_5: neighbors trap0" );
-		myQs.check_trapezoid_neighbors(  1, 0, null, 2, null, "Spec_5: neighbors trap1" );
-		myQs.check_trapezoid_neighbors(  2, 1, null, 4, null, "Spec_5: neighbors trap2" );
-		myQs.check_trapezoid_neighbors(  3, null, 0, null, 5, "Spec_5: neighbors trap3" );
-		myQs.check_trapezoid_neighbors(  4, 2, 5, 6, 8, "Spec_5: neighbors trap4" );
-		myQs.check_trapezoid_neighbors(  5, null, 3, null, 4, "Spec_5: neighbors trap5" );
-		myQs.check_trapezoid_neighbors(  6, 4, null, 7, null, "Spec_5: neighbors trap6" );
-		myQs.check_trapezoid_neighbors(  7, 6, 8, null, null, "Spec_5: neighbors trap7" );
-		myQs.check_trapezoid_neighbors(  8, null, 4, null, 7, "Spec_5: neighbors trap8" );
-
-//		var myQsRoot = myQs.getRoot();
-//		showDataStructure( myQsRoot );
-//		drawTrapezoids( myQsRoot, false, 0.4 );
-	}
-
-	function test_add_segment_special_6() {
-
 		var myPolygonData = new PNLTRI.PolygonData( [ [
 			{ x:43, y:31 }, { x:41, y:29 },
 			{ x:42, y:40 }, { x:36, y:24 },
@@ -1901,7 +2063,7 @@ function test_QueryStructure() {
 //		drawTrapezoids( myQsRoot, false, 1 );
 	}
 
-	function test_add_segment_special_7() {
+	function test_add_segment_special_6() {
 
 		var myPolygonData = new PNLTRI.PolygonData( [ [
 			{ x:10, y:34 }, { x:36, y:12 }, { x:32, y:20 }, { x:28, y:24 },
@@ -1990,8 +2152,9 @@ function test_QueryStructure() {
 		//
 		test_ptNode();
 		test_ptNode_touching();
-		test_ptNode_colinear_1();
-		test_ptNode_colinear_2();
+		test_ptNode_colinear_inside();
+		test_ptNode_colinear_overlapping();
+		test_ptNode_colinear_reversal();
 		//
 		test_assign_depths();
 		//
@@ -2013,9 +2176,8 @@ function test_QueryStructure() {
 		test_add_segment_special_2();
 		test_add_segment_special_3();
 		test_add_segment_special_4();
-//		test_add_segment_special_5();		// co-linear removed on input
+		test_add_segment_special_5();
 		test_add_segment_special_6();
-		test_add_segment_special_7();
 		// for testing new polygons
 //		test_add_segment_NEW();
 //		test_add_segment_Error();