Function mix_in_selector 

pub fn mix_in_selector<D: Digest<OutputSize = U32>>(
    root: [u8; 32],
    selector: u8,
) -> [u8; 32]

mix_in_selector(root, sel) = hash(root || u256_le(sel)).

Note that the selector is padded to a full 32-byte little-endian u256, matching the spec (not just one byte).