Function richelot_chain 

pub fn richelot_chain(
    roots: &[BigInt; 6],
    depth: usize,
    p: &BigInt,
) -> Option<Vec<usize>>

Walk chains of Richelot (2,2)-isogenies to depth depth (a chain of depth + 1 steps), returning the sequence of neighbour indices that first reaches a split (reducible) surface, or None if none does within the bound. This is the recursive form of the Castryck–Decru search across the (2,2)-graph: from each node it forms the codomain, extracts its Weierstrass points, and recurses. Neighbours whose codomain 2-torsion escapes to 𝔽_{p²} are skipped over the prime field.