This pari program can be used, in conjunction with the tables
of eigenforms, to compute the degree d relations satisfied
by cusp forms in
S_{2}(_{0}(N)).
The algorithm is described in Steven Galbraith's Oxford Ph.D. thesis.
The implimentation (written in one afternoon) is not efficient, but
it was good enough for the application of determining all trigonal
modular curves, for which it was written.