Function decrypt_block 

pub fn decrypt_block<W: Word>(
    expanded_key: &[W],
    block: [W; 2],
) -> Result<[W; 2], Error>

Block Decryption

The decryption routine is easily derived from the encryption routine. for i=r downto 1 do B = ((B − S[2 ∗ i + 1]) > A) ⊕ A; A = ((A − S[2 ∗ i]) > B) ⊕ B;

B = B − S[1]; A = A − S[0];