We can associate a theta series to any pair of elements of a Brandt module, which give embeddings (with respect to any fixed module element) of the Brandt module in a space of weight 2 modular forms.
Returns the theta series associated to the pair (x, y) of elements of a Brandt module, as an element of a power series ring.
A sequence of power series elements, to precision prec, spanning the image of the theta functions associated to pairs in the Brandt module M.