Assuming we don't change the meaning (or truth table) of $\displaystyle \Leftrightarrow$, is it true that the truth table for $\displaystyle \Rightarrow$ is the only one which makes:

$\displaystyle ((a \Rightarrow b) \land (b \Rightarrow a)) \Leftrightarrow (a \Leftrightarrow b)$

a tautology? Incidentally, what is the translation of this statement back to the language of $\displaystyle \cup, \cup \ and \subset$