The implementation currently assumes that the congruence subgroup is SL_2(Z) and that the ring is generated by E2, E4 and E6. However, it is possible to create a ring for a congruence subgroup, which leads to some errors like this:

sage: QM = QuasiModularForms(Gamma0(17))
sage: f = QM.0
sage: f.to_polynomial()
---------------------------------------------------------------------------
Traceback (most recent call last):
...
ValueError: too many values to unpack (expected 2)

The goal of this ticket is to make the code for quasimodular forms ring coherent for congruence subgroup and add more doctests.

CC: @videlec

Component: modular forms

Keywords: quasiforms to_polynomial

Author: David Ayotte

Branch/Commit: 51412f3

Reviewer: Vincent Delecroix

Issue created by migration from https://trac.sagemath.org/ticket/34569