Function tuplehash256 

pub fn tuplehash256(tuple: &[&[u8]], custom: &[u8], out: &mut [u8])

TupleHash256 (NIST SP 800-185 §5).

Each element of tuple is length-prefixed with encode_string, so the digest is unambiguous over the tuple structure. NIST SP 800-185 §5.3.