Function xof_request 

pub fn xof_request(seed: &[u8; 32], block: u8) -> Request

Xof(t): output block t of BLAKE3-XOF(seed).

BLAKE3’s extendable output: the same root compression run with counter t = 0, 1, 2, … gives 64 fresh bytes each time. 20 blocks give the 1,224 bytes of chain secrets (68 × 18).