要写好几步(p→q)<->r<=>((┐p∨q)→r)∧(r→(┐p∨q))<=>(┐(┐p∨q)∨r)∧(┐r∨(┐p∨q))<=>((p∧┐q)∨r)∧(┐p∨q∨┐r)<=>((p∧r)∨(┐q∧r))∧(┐p∨q∨┐r)
