Function derive_isogeny_path 

pub fn derive_isogeny_path(
    curve: &Curve,
    gen: &Point,
    ell: u64,
    a: u32,
) -> Option<Vec<IsogenyStep>>

Torsion-image → path derivation. A degree-ℓᵃ isogeny is pinned down by a single kernel generator gen of order ℓᵃ — the datum the SIDH/SIKE torsion images reconstruct (via Kani’s gluing). This unfolds that one generator into the explicit chain of a prime-degree ℓ-isogenies: at step i the kernel is the order-ℓ point [ℓ^{a−1−i}]·genᵢ, and gen is pushed forward through each step so its order descends ℓᵃ → ℓᵃ⁻¹ → … → 1. The entire secret path pops out of the one meta-datum — the generator is the rule that emits the per-step rules. Requires ℓ an odd prime and gen of order exactly ℓᵃ.