Fix of MinimalHereditarySubsetsVertex filter

[joe-dw]

MinimalHereditarySubsetsVertex should take an input of a digraph and integer:

if IsMultiDigraph(D) then ErrorNoReturn("the 1st argument (a digraph) must not have multiple edges");

But it was previously defined with the filter

DeclareOperation("MinimalHereditarySubsetsVertex", [IsGraphInverseSemigroup, IsPosInt]);

I have changed this to require a digraph instead of a GIS

[james-d-mitchell]

As discussed, this is probably not the correct place to make this change. I suggest doing the following:

1. Move the declaration ofMinimalHereditarySubsetsVertex to Digraphs
2. Copy the function SEMIGROUPS_MinimalHereditarySubsetsVertex as the installed method for MinimalHereditarySubsetsVertex in Digraphs
3. Modify the Semigroups gi file to look (something) like the following (but that actually works in case this does not):

if not IsBoundGlobal("MinimalHereditarySubsetsVertex") then
    BindGlobal("SEMIGROUPS_MinimalHereditarySubsetsVertex",
    function(D, v)
    local subsets, hereditary, u, out, s, a;
    if IsMultiDigraph(D) then
        ErrorNoReturn("the 1st argument (a digraph) must not have multiple edges");
    elif not (v in DigraphVertices(D)) then
        ErrorNoReturn("the 2nd argument (a pos. int.) is not a vertex of ",
                    "the 1st argument (a digraph)");
    fi;
    out := Set(OutNeighboursMutableCopy(D)[v]);
    subsets := [];
    for u in [1 .. Length(out)] do
        a := out[u];
        RemoveSet(out, a);
        hereditary := ShallowCopy(out);
        for s in out do
        UniteSet(hereditary, VerticesReachableFrom(D, s));
        od;
        if not (a in hereditary) and not (hereditary in subsets) then
        AddSet(subsets, hereditary);
        fi;
        AddSet(out, a);
    od;
    return AsSortedList(subsets);
    end);
else
    BindGlobal("SEMIGROUPS_MinimalHereditarySubsetsVertex",
    MinimalHereditarySubsetsVertex);
fi;