Bug in NullspaceModQ

[fingolfin]

The following was reported on https://math.stackexchange.com/questions/3329100:

gap> NullspaceModQ([[2]],8);
[ [ 0 ] ]

But it really should return the list [ [0], [4] ].

[fingolfin]

One option would be to just ditch the current implementation and use this code instead (which then also works for arbitrary moduli):

NullspaceModN := function(M, n)
  local B, coeffs;
  B := BasisNullspaceModN(M, n);
  coeffs := Cartesian(ListWithIdenticalEntries(Length(B), [0..n-1]));
  return Set(coeffs, c -> c * B mod n);
end;

Alas, perhaps NullspaceModQ is faster in some cases? One would have to conduct experiments to find out (although fast but incorrect code is not that useful...).

Of course one could also instead improve BasisNullspaceModN and perhaps even make a BasisNullspaceModQ, which strikes me as more useful than NullspaceModQ anyway, at least for most purposes.

[hulpke]

I am not sure whether NullspaceModQ is used anywhere. I would be happy with ditching the function completely (or replace it with the proposed code and move it on the way to obsolescence.