gh-36988: Adding support for sums, intersection, and equality of SubmodulesWithBasis
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Common vector subspace operations are taking sums, intersections, and
checking equality, but these are not implemented (inclusion already is).
We provide generic implementations of these for objects in
`ModulesWithBasis` and make a few other minor improvements.
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URL: https://github.com/sagemath/sage/pull/36988
Reported by: Travis Scrimshaw
Reviewer(s): Martin Rubey, Travis Scrimshaw
