Function xadd 

pub fn xadd(
    xp: &BigInt,
    zp: &BigInt,
    xq: &BigInt,
    zq: &BigInt,
    xd: &BigInt,
    zd: &BigInt,
    n: &BigInt,
) -> (BigInt, BigInt)

x-only differential addition on a Montgomery curve mod n: given P=(Xp:Zp), Q=(Xq:Zq) and their known difference P−Q = (Xd:Zd), return P+Q. Again inversion-free.