Function: mfbd
Section: modular_forms
C-Name: mfbd
Prototype: GL
Help: mfbd(F,d): F being a generalized modular form, return B(d)(F), where
 B(d) is the expanding operator tau -> d tau.
Doc: $F$ being a generalized modular form, return $B(d)(F)$, where $B(d)$ is
 the expanding operator $\tau\mapsto d\tau$.
 \bprog
 ? D2=mfbd(mfDelta(),2); mfcoefs(D2, 6)
 %1 = [0, 0, 1, 0, -24, 0, 252]
 @eprog
