in fails to find the zero element in a partition of a graph inverse semigroup (issue #6374 on gap-system/gap)

[joe-dw]

The following example shows an issue where = is able to recognise that the zero element is equal to an element of the semigroup, but in is not. I have not been able to find a smaller or simpler digraph where this happens

gap> D := Digraph([[4,5],[1,3,4,5],[1,4,5],[],[4]]);
<immutable digraph with 5 vertices, 10 edges>
gap> G := GraphInverseSemigroup(D);
<finite graph inverse semigroup with 5 vertices, 10 edges>
gap> c := CongruencesOfSemigroup(GraphInverseSemigroup(D))[3];
<graph inverse semigroup congruence with H = [ 1, 4, 5 ] and W = [ 2 ]>
gap> p := EquivalenceRelationPartitionWithSingletons(c);;
gap> p[1][22] = MultiplicativeZero(G);
true
gap> MultiplicativeZero(G) in p[1];
false

[joe-dw]

The issue also appears for everything in Elements(G) for this particular digraph:

gap> List(Elements(G),g -> ForAny(p,s -> g in s));
returns
[ false, ... ,false ]

but each element also correctly returns true if compared to an individual element within the partition

[james-d-mitchell]

Yikes, something has clearly gone rather wrong there. I'll investigate, thanks for the report @joe-dw