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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def instructions(self):
 """ The number of instructions in this block """

        if self._instructions is None:
            if self.statements is None:
                self._instructions = 0
            else:
                self._instructions = len([s for s in self.statements if type(s) is stmt.IMark])
        return self._instructions