Function is_refutation 

pub fn is_refutation(
    equations: &[ModpEquation],
    num_vars: usize,
    p: u64,
    combo: &[(usize, u64)],
) -> bool

Re-check a refutation: the chosen combination of equations has every variable coefficient ≡ 0 and a nonzero right-hand side mod p — a solver-free certificate of inconsistency.