semirel: fix FroidurePinExtended for pperm monoids

Resolves #1674

lib/semirel.gi

@@ -959,7 +959,7 @@ function(m)
   local gens, k, free, freegens, actualelts, fpelts, rules, i, u, v, Last,
         currentlength, b, s, r, newelt, j, p, new, length, newword, first,
         final, prefix, suffix, next, postmult, reducedflags, premult, fpsemi,
-        old, sortedelts, pos, semi, perm, free2;
+        old, sortedelts, pos, semi, perm, free2, one;
 
   if not IsFinite(m) then 
     return fail;
@@ -972,13 +972,14 @@ function(m)
   fi;
 
   #gens:=Set(GeneratorsOfMonoid(semi));
-  gens:=Set(Filtered(GeneratorsOfMonoid(semi), x-> not IsOne(x)));
+  one:=One(semi);
+  gens:=Set(Filtered(GeneratorsOfMonoid(semi), x -> x <> one));
   k:=Length(gens);
   free:=FreeMonoid(k);
   freegens:=GeneratorsOfMonoid(free);
-  actualelts:=Concatenation([One(semi)], gens);
+  actualelts:=Concatenation([one], gens);
   fpelts:=Concatenation([One(free)], freegens);
-  sortedelts:=List(Concatenation([One(semi)], gens));
+  sortedelts:=List(Concatenation([one], gens));
 
   #output
 

tst/testbugfix/2017-09-07-FroidurePinExtendedAlg.tst

@@ -0,0 +1,48 @@
+# Issue related to FroidurePinExtendedAlg
+# Examples reported on issue #1674 on github.com/gap-system/gap
+#
+gap> sort := function(x, y)
+> local rx, ry;
+> rx := RankOfPartialPerm(Representative(x));
+> ry := RankOfPartialPerm(Representative(y));
+> return rx < ry;
+> end;;
+gap> x := PartialPerm([1]);
+<identity partial perm on [ 1 ]>
+gap> y := PartialPerm([0]);
+<empty partial perm>
+gap> S := Semigroup(x, y);
+<partial perm monoid of rank 1 with 2 generators>
+gap> D := ShallowCopy(GreensDClasses(S));;
+gap> Sort(D, sort);
+gap> D;
+[ <Green's D-class: <empty partial perm>>, 
+  <Green's D-class: <identity partial perm on [ 1 ]>> ]
+gap> Elements(S);
+[ <empty partial perm>, <identity partial perm on [ 1 ]> ]
+gap> S := Semigroup(x, y);
+<partial perm monoid of rank 1 with 2 generators>
+gap> Elements(S);
+[ <empty partial perm>, <identity partial perm on [ 1 ]> ]
+gap> D := ShallowCopy(GreensDClasses(S));;
+gap> Sort(D, sort);
+gap> D;
+[ <Green's D-class: <empty partial perm>>, 
+  <Green's D-class: <identity partial perm on [ 1 ]>> ]
+
+#
+gap> SymmetricInverseMonoid(2);
+<symmetric inverse monoid of degree 2>
+gap> D := ShallowCopy(GreensDClasses(last));;
+gap> Sort(D, sort);
+gap> D;
+[ <Green's D-class: <empty partial perm>>, 
+  <Green's D-class: <identity partial perm on [ 1 ]>>, 
+  <Green's D-class: <identity partial perm on [ 1, 2 ]>> ]
+
+#
+gap> S := Semigroup(x, y);
+<partial perm monoid of rank 1 with 2 generators>
+gap> FroidurePinExtendedAlg(S);
+gap> LeftCayleyGraphSemigroup(S);
+[ [ 1 ], [ 1 ] ]