semigrp.gi: add One to GeneratorsOfMagma if needed

Resolves semigroups#371
wilfwilson · Sep 13, 2017 · 6a28c68 · 6a28c68
1 parent 74b64e2
commit 6a28c68
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Showing 2 changed files with 14 additions and 3 deletions.
diff --git a/gap/semigroups/semigrp.gi b/gap/semigroups/semigrp.gi
@@ -277,10 +277,10 @@ function(gens, opts)
 
   S := Objectify(NewType(FamilyObj(gens), filts), rec(opts := opts));
 
-  if CanEasilyCompareElements(gens) and not One(gens) in gens then
-    SetGeneratorsOfMagma(S, Concatenation([One(gens)], gens));
-  else
+  if One(gens) in gens then
     SetGeneratorsOfMagma(S, AsList(gens));
+  else
+    SetGeneratorsOfMagma(S, Concatenation([One(gens)], gens));
   fi;
   SetGeneratorsOfMagmaWithOne(S, AsList(mgens));
 

diff --git a/tst/testinstall.tst b/tst/testinstall.tst
@@ -1656,6 +1656,17 @@ gap> S := SingularFactorisableDualSymmetricInverseMonoid(3);
 gap> IsMonoidAsSemigroup(S);
 false
 
+#T# Issue 371: GeneratorsOfSemigroup for a monoid
+gap> R := ReesMatrixSemigroup(Group(()), [[()]]);;
+gap> S := MonoidByAdjoiningIdentity(R);
+<commutative monoid with 1 generator>
+gap> GeneratorsOfSemigroup(S);
+[ ONE, (1,(),1) ]
+gap> Size(S);
+2
+gap> Elements(S);
+[ ONE, (1,(),1) ]
+
 #T# SEMIGROUPS_UnbindVariables
 gap> Unbind(B);
 gap> Unbind(D);