Function reynolds_gfp 

pub fn reynolds_gfp(
    f: NsField,
    l: &[(Mono, u64)],
    group: &[Perm],
) -> Option<Vec<(Mono, u64)>>

The Reynolds operator L ↦ |G|⁻¹ · Σ_{g∈G} g·L over f — the characteristic-0 averaging trick, available exactly when |G| is invertible, i.e. gcd(|G|, char) = 1. Returns None when char | |G| (the annihilation branch: the group-sum kills L(1) and no normalization can restore it), and the averaged functional otherwise. Averaging a valid pseudo-expectation of a G-invariant generator system yields a valid, G-invariant one — the branch the GF(2) engine can never take on an even group. Pass the full closed group.