Function tuplehash256_xof 

pub fn tuplehash256_xof(tuple: &[&[u8]], custom: &[u8], out: &mut [u8])

TupleHashXOF256 (NIST SP 800-185 §5).

XOF variant: right_encode(0) in place of the output length. NIST SP 800-185 §5.3.1.