Matrix_integer_sparse object has no attribute 'gap'

[slabbe]

sage: import itertools
sage: G = [w.matrix() for w in WeylGroup(['B', 3])]
sage: G = MatrixGroup(G)
sage: H = [diagonal_matrix(L) for L in itertools.product((1,-1), repeat=3)]
sage: H = G.subgroup(H)
Traceback (most recent call last):
...
AttributeError: 'sage.matrix.matrix_integer_sparse.Matrix_integer_sparse' object has no attribute 'gap'

[tscrim]

None of the matrices have a gap method.

The code is assuming that the input are elements of G; so it is bad input. However, to make things more robust and better behaved, I would make this change (the cost of the coercion call should be minimal):

-generators = [ g if isinstance(g, GapElement) else g.gap()
+generators = [ g if isinstance(g, GapElement) else self(g).gap()
                for g in generators ]

[slabbe]

Sorry for taking so long.

---

New commits:

​15b13f9

19720: more robust ParentLibGAP.subgroup method

[tscrim]

LGTM. Thanks.