FIX: MTX will do better work in deleting superfluous generators.

This resolves #4406
gap-system · Apr 16, 2021 · 9ade12b · 9ade12b
1 parent b435ff1
commit 9ade12b
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Showing 2 changed files with 9 additions and 2 deletions.
diff --git a/lib/ghomfp.gi b/lib/ghomfp.gi
@@ -871,7 +871,10 @@ local aug,w,p,pres,f,fam,G,trace;
 
     pres:=NEWTC_PresentationMTC(aug,0,nam);
   fi;
-  Assert(0,Length(GeneratorsOfPresentation(pres))=Length(gens));
+  # check that we have the exat generators as we want and no rearrangement
+  # or so happened.
+  Assert(0,Length(GeneratorsOfPresentation(pres))=Length(gens) 
+    and pres!.primarywords=[1..Length(gens)]);
 
   # new free group
   f:=FpGroupPresentation(pres);

diff --git a/lib/sgpres.gi b/lib/sgpres.gi
@@ -3585,12 +3585,16 @@ local DATA,rels,i,j,w,f,r,s,fam,new,ri,a,offset,p,rset,re,start,stack,pres,
       od;
       r:=AssocWordByLetterRep(fam,Concatenation(r,[-i]));
       AddRelator(pres,r);
-      TzSearch(pres);
+      #TzSearch(pres); Do *not* search, as this might kill the relator we
+      #just added.
       TzEliminate(pres,i);
     od;
     Assert(1,Length(GeneratorsOfPresentation(pres))=subnum);
 
   fi;
+  r:=List(GeneratorsOfPresentation(pres){[1..subnum]},
+    x->LetterRepAssocWord(x)[1]);
+  pres!.primarywords:=r;
   r:=List(GeneratorsOfPresentation(pres){
       [subnum+1..Length(GeneratorsOfPresentation(pres))]},
         x->LetterRepAssocWord(x)[1]);