polmodular [1].polmodular(3,5) is correspondingly undefined.\\ prime level 11, invariant 5 = gamma_2; PARI/GP prints the polynomial nested in x
polmodular(11, 5)The generator computes exact $q$-expansions of $\gamma_2(\tau)$ and $\gamma_2(\ell\tau)$ from the branch in Formula (1) and finds the one-dimensional rational kernel among monomials $x^a y^b$ with $0\leq a,b\leq\ell+1$ and $\ell a+b\equiv\ell+1\pmod 3$. For the rows in this table, the resulting polynomial is irreducible over $\mathbb Q(y)$.
The result is primitive over $\mathbb Z$, has leading coefficient $1$ in $x^{\ell+1}$, and is compared with PARI/GP's exact polmodular(ell,5) output [1].