Function is_refutation 

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

Re-check a refutation over ℤ/modulus: the chosen combination of equations has every variable coefficient ≡ 0 and a nonzero right-hand side mod modulus — a solver-free certificate that the system is inconsistent over that quotient ring (and hence over any ℤ/m with modulus | m).