<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def size(self):
 """ Recursively find size of a tree. Slow. """


        if self is NULL:
            return 0
        return 1 + self.left.size() + self.right.size()