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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 _out(self, allow_placeholders):
 """ Converts the Quil program to a readable string. :param allow_placeholders: Whether to complain if the program contains placeholders. """

        return '\n'.join(itertools.chain(
            (dg.out() for dg in self._defined_gates),
            (instr.out(allow_placeholders=allow_placeholders) for instr in self.instructions),
            [''],
        ))