Proof Writing & Problem Solving · Superchallenge problem

Using only the letters $P$ and $Q$, parentheses, and the connective $\Rightarrow$ (as many times as you like), one can build statements such as $P \Rightarrow (Q \Rightarrow P)$ or $(P \Rightarrow Q) \Rightarrow Q$. Two statements count as the same if they have the same truth table. How many different truth tables can be produced?