(p←→q) <=> (p→q)∧(q→p) <=> (┐p∨q)∧(┐q∨p) <=> (┐p∧┐q)∨ (┐p∧p)∨(q∧┐q)∨(q∧p)<=> (┐p∧┐q)∨(q∧p)<=> ┐(p∨q)∨(q∧p) => ┐(p∨q)
