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Complement of old_submodule gives strange error


I've been finding a weird error while trying to compute the complement to a p-new subspace of a space of modular forms. Running:


raises "ArithmeticError: vector is not in free module", and I suspect it's only because it tries to generate intersect a Hecke module with a 0-dimensional thing, or because it fails to compute some q-expansion basis.

Any thoughts on how to fix this?

Running: 'SageMath version 9.5, Release Date: 2022-01-30'

Edit: it seems to be raised by running ModularForms(88).old_submodule(8).hecke_polynomial(2) Changing the .complement() method to start with hecke_polynomial(3) kind of fixes this, but forces it to fall back on the naive algorithm, which eventually gives a NotImplementedError.