Bug in acting semigroups ("<func> must return a value")

[flsmith]

@wilfwilson wrote in Slack

"I get the following error when stretching the acting code to its limits:

gap> PartialOrderOfDClasses(MotzkinMonoid(13));
Error, Function Calls: <func> must return a value in
 AddSet( out[i], lookup[Position( o, lambdafunc( y * x ) )] - 1
); at /Users/Wilf/GAP/pkg/semigroups/gap/greens/gracreg.gi:305 called from
<function "unknown">( <arguments> )
called from read-eval loop at *stdin*:23
you can supply one by 'return <value>;'

If it doesn’t work with you with MotzkinMonoid(13), maybe it would with 14 or higher. I’ve encountered simliar problems with JonesMonoid(21) and AnnularJonesMonoid(22)"

I can reproduce this error reliably.

(Possible red herring) I also suspect that the same bug is causing an issue I'm seeing in my dual semigroups code, which boils down to the following example. The error triggers within about 20s on my computer.

gap> S := SingularPartitionMonoid(5);;
gap> while true do
> List(S, x -> x in S);;
> od;
Error, Function Calls: <func> must return a value in
  xx := LambdaFunc( I )( x )
 ; at /home/finn/gap/pkg/semigroups/gap/ideals/idealact.gi:778 called from 
x in S at *stdin*:3 called from
func( elm ) at /home/finn/gap/lib/coll.gi:796 called from
CallFuncList( ListOp, arg ) at /home/finn/gap/lib/coll.gi:758 called from
List( S, function ( x )
      return x in S;
  end ); at *stdin*:3 called from
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:4
you can supply one by 'return <value>;'

@markuspf suggested that this was likely a garbage collection related bug. The following code tends not to error (at least within any reasonable time), but I have seen the same error as before occur with this code too.

gap> S := SingularPartitionMonoid(5);;
gap> while true do
> List(S, x -> x in S);;
> GASMAN("collect");
> od;

[ssiccha]

I get a similar error when running the 'extreme' test directory.

[james-d-mitchell]

Fixed by #466