FIX: Overlooked case for cyclic MTC (#5532)

When rewriting to cyclic subgroup words are just numbers, not lists. Thus
concatenation needs to be replaced by addition. This was overlooked in one
case.

Co-authored-by: Alexander Hulpke <hulpke@math.colostate.edu>

lib/sgpres.gi

@@ -2847,7 +2847,12 @@ local c,offset,f,b,r,i,j,fp,bp;
     od;
     if i>r then
       if f<>alpha then
-        NEWTC_ModifiedCoincidence(DATA,f,alpha,WordProductLetterRep(-Reversed(fp),y));
+        if DATA.useAddition then
+          NEWTC_ModifiedCoincidence(DATA,f,alpha,-fp+y);
+        else
+          NEWTC_ModifiedCoincidence(DATA,f,alpha,
+            WordProductLetterRep(-Reversed(fp),y));
+        fi;
       fi;
       return;
     fi;

tst/testbugfix/2023-10-28-GQuotients.tst

@@ -0,0 +1,7 @@
+# Fix unexpected error in GQuotients.
+# See https://github.com/gap-system/gap/issues/5525
+#
+gap> G := FreeGroup(2);;
+gap> Q := G / [G.1*G.2^-2*G.1, G.1^-1*G.2^-3];;
+gap> GQuotients(Q, SmallGroup(12, 1));
+[  ]