<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):
 """ The size of this block, in bytes """

        if self._size is None:
            self._size = sum(s.len for s in self.statements if type(s) is stmt.IMark)
        return self._size