((x v y) <-> (~x -> ~y))
= (x v y) <-> (~ (~x) v ~y)
= (x v y) <-> (x v ~y)
= ((x v y) ^ (x v ~y)) v (~(x v y) ^ ~(x v ~y))
= ((x v (y ^ ~y)) v ((~x ^ ~y) ^ (~x ^ y))
= (x v F) v ((~x ^ ~y) ^ (~x ^ y))
= x v (~x ^ (y ^ ~y)) = x v (~x ^ F)
= x v F
=x