diff --git a/mesh/MeshEditor.cs b/mesh/MeshEditor.cs
index 0b840ee6..89d66585 100644
--- a/mesh/MeshEditor.cs
+++ b/mesh/MeshEditor.cs
@@ -84,8 +84,8 @@ public virtual int[] AddTriangleFan_OrderedEdgeLoop(int center, int[] edge_loop,
             operation_failed:
                 // remove what we added so far
                 if (i > 0) {
-                    if (remove_triangles(new_tris, i) == false)
-                        throw new Exception("MeshConstructor.AddTriangleFan_OrderedEdgeLoop: failed to add fan, and also falied to back out changes.");
+                    if (remove_triangles(new_tris, i-1) == false)
+                        throw new Exception("MeshConstructor.AddTriangleFan_OrderedEdgeLoop: failed to add fan, and also failed to back out changes.");
                 }
                 return null;
         }
@@ -94,6 +94,48 @@ public virtual int[] AddTriangleFan_OrderedEdgeLoop(int center, int[] edge_loop,
 
 
 
+        public virtual int[] StitchLoop(int[] vloop1, int[] vloop2, int group_id = -1)
+        {
+            int N = vloop1.Length;
+            if (N != vloop2.Length)
+                throw new Exception("MeshEditor.StitchLoop: loops are not the same length!!");
+
+            int[] new_tris = new int[N * 2];
+
+            int i = 0;
+            for ( ; i < N; ++i ) {
+                int a = vloop1[i];
+                int b = vloop1[(i + 1) % N];
+                int c = vloop2[i];
+                int d = vloop2[(i + 1) % N];
+
+                Index3i t1 = new Index3i(b, a, d);
+                Index3i t2 = new Index3i(a, c, d);
+
+                int tid1 = Mesh.AppendTriangle(t1, group_id);
+                int tid2 = Mesh.AppendTriangle(t2, group_id);
+
+                if (tid1 == DMesh3.InvalidID || tid2 == DMesh3.InvalidID)
+                    goto operation_failed;
+
+                new_tris[2 * i] = tid1;
+                new_tris[2 * i + 1] = tid2;
+            }
+
+            return new_tris;
+
+
+            operation_failed:
+                // remove what we added so far
+                if (i > 0) {
+                    if (remove_triangles(new_tris, 2*(i-1)) == false)
+                        throw new Exception("MeshConstructor.StitchLoop: failed to add all triangles, and also failed to back out changes.");
+                }
+                return null;
+        }
+
+
+
 
 
         // [TODO] cannot back-out this operation right now
diff --git a/mesh_ops/MeshExtrusion.cs b/mesh_ops/MeshExtrusion.cs
new file mode 100644
index 00000000..979b0ee0
--- /dev/null
+++ b/mesh_ops/MeshExtrusion.cs
@@ -0,0 +1,71 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace g3
+{
+    public class MeshExtrusion
+    {
+        public DMesh3 Mesh;
+        public EdgeLoop Loop;
+
+        // arguments
+
+        // set new position based on original loop vertex position, normal, and index
+        public Func<Vector3d, Vector3f, int, Vector3d> PositionF;
+
+        // outputs
+
+        public int[] NewTriangles;
+        public EdgeLoop NewLoop;
+
+
+        public MeshExtrusion(DMesh3 mesh, EdgeLoop loop)
+        {
+            Mesh = mesh;
+            Loop = loop;
+
+            PositionF = (pos, normal, idx) => {
+                return pos + Vector3d.AxisY;
+            };
+        }
+
+
+        public virtual ValidationStatus Validate()
+        {
+            ValidationStatus loopStatus = MeshValidation.IsBoundaryLoop(Mesh, Loop);
+            return loopStatus;
+        }
+
+
+        public virtual bool Extrude(int group_id = -1)
+        {
+            // duplicate loop vertices
+            int NV = Loop.Vertices.Length;
+            NewLoop = new EdgeLoop(Mesh);
+            NewLoop.Vertices = new int[NV];
+
+            for ( int i = 0; i < NV; ++i ) {
+                int vid = Loop.Vertices[i];
+                NewLoop.Vertices[i] = Mesh.AppendVertex(Mesh, vid);
+            }
+
+            // move to offset positions
+            for ( int i = 0; i < NV; ++i ) {
+                Vector3d v = Mesh.GetVertex(Loop.Vertices[i]);
+                Vector3f n = Mesh.GetVertexNormal(Loop.Vertices[i]);
+                Vector3d new_v = PositionF(v, n, i);
+                Mesh.SetVertex(NewLoop.Vertices[i], new_v);
+            }
+
+            // stitch interior
+            MeshEditor edit = new MeshEditor(Mesh);
+            NewTriangles = edit.StitchLoop(Loop.Vertices, NewLoop.Vertices, group_id);
+
+            return true;
+        }
+
+
+    }
+}
diff --git a/mesh_ops/SimpleHoleFiller.cs b/mesh_ops/SimpleHoleFiller.cs
index 18b3bb87..53cad00e 100644
--- a/mesh_ops/SimpleHoleFiller.cs
+++ b/mesh_ops/SimpleHoleFiller.cs
@@ -10,8 +10,8 @@ public class SimpleHoleFiller
         public DMesh3 Mesh;
         public EdgeLoop Loop;
 
-        int NewVertex;
-        int[] NewTriangles;
+        public int NewVertex;
+        public int[] NewTriangles;
 
 
         public SimpleHoleFiller(DMesh3 mesh, EdgeLoop loop)
@@ -24,7 +24,15 @@ public SimpleHoleFiller(DMesh3 mesh, EdgeLoop loop)
         }
 
 
-        public bool Fill(int group_id = -1)
+
+        public virtual ValidationStatus Validate()
+        {
+            ValidationStatus loopStatus = MeshValidation.IsBoundaryLoop(Mesh, Loop);
+            return loopStatus;
+        }
+
+
+        public virtual bool Fill(int group_id = -1)
         {
             if (Loop.Vertices.Length < 3)
                 return false;
diff --git a/queries/MeshValidation.cs b/queries/MeshValidation.cs
new file mode 100644
index 00000000..4d4fc1de
--- /dev/null
+++ b/queries/MeshValidation.cs
@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+
+
+namespace g3
+{
+    public enum ValidationStatus
+    {
+        Ok,
+
+        NotBoundaryVertex,
+        NotBoundaryEdge,
+
+        VerticesNotConnectedByEdge,
+        IncorrectLoopOrientation
+    }
+
+
+    public static class MeshValidation
+    {
+
+        public static ValidationStatus IsBoundaryLoop(DMesh3 mesh, EdgeLoop loop)
+        {
+            int N = loop.Vertices.Length;
+
+            for ( int i = 0; i < N; ++i ) {
+                if ( ! mesh.vertex_is_boundary(loop.Vertices[i]) )
+                    return ValidationStatus.NotBoundaryVertex;
+            }
+
+            for ( int i = 0; i < N; ++i ) {
+                int a = loop.Vertices[i];
+                int b = loop.Vertices[(i + 1) % N];
+
+                int eid = mesh.FindEdge(a, b);
+                if (eid == DMesh3.InvalidID)
+                    return ValidationStatus.VerticesNotConnectedByEdge;
+
+                if (mesh.edge_is_boundary(eid) == false)
+                    return ValidationStatus.NotBoundaryEdge;
+
+                Index2i ev = mesh.GetOrientedBoundaryEdgeV(eid);
+                if (!(ev.a == a && ev.b == b))
+                    return ValidationStatus.IncorrectLoopOrientation;
+            }
+
+            return ValidationStatus.Ok;
+        }
+
+
+    }
+}