Function ecm_two_stage 

pub fn ecm_two_stage(
    n: &BigInt,
    b1: u64,
    b2: u64,
    curves: usize,
    seed: u64,
) -> Option<BigInt>

ECM with stage 1 + stage 2. Stage 2 (bound b2 > b1) extends each curve’s reach to a group order that is b1-smooth apart from one prime ≤ b2 — a large boost per curve for a small extra cost.