Get a particular instruction by using (default) the index of the address if specified

        :param idx: index of the instruction (the position in the list of the instruction)
        :type idx: int
        :param off: address of the instruction
        :type off: int

        :rtype: an :class:`Instruction` object

def get_instruction(self, idx, off=None):
        """
        Get a particular instruction by using (default) the index of the address if specified

        :param idx: index of the instruction (the position in the list of the instruction)
        :type idx: int
        :param off: address of the instruction
        :type off: int

        :rtype: an :class:`Instruction` object
        """
        if off is not None:
            idx = self.off_to_pos(off)
        if self.cached_instructions is None:
            self.get_instructions()
        return self.cached_instructions[idx]