Fix corner case in modified Todd-Coxeter when relator is trivial

gap-system · Feb 20, 2019 · ce0ad33 · ce0ad33
1 parent 08109a0
commit ce0ad33
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Showing 2 changed files with 6 additions and 0 deletions.
diff --git a/lib/sgpres.gi b/lib/sgpres.gi
@@ -2904,6 +2904,7 @@ local m,offset,rels,ri,ccr,i,r,ct,A,a,w,n,DATA,p,ds,dr,
   fi;
   offset:=m+1;
   rels:=ShallowCopy(freerels);
+  rels:=Filtered(freerels, x -> Length(x) > 0);
   SortBy(rels,Length);
   ri:=Union(rels,List(rels,x->x^-1));
   ri:=List(ri,LetterRepAssocWord);

diff --git a/tst/testbugfix/2019-02-19-cornercase-mtc.tst b/tst/testbugfix/2019-02-19-cornercase-mtc.tst
@@ -0,0 +1,5 @@
+gap> F := FreeGroup(1);;
+gap> rels := [F.1^2, One(F)];;
+gap> G := F / rels;;
+gap> Order(G);
+2