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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 _synthesize(self):
 """ Assigns all placeholder labels to actual values and implicitly declares the ``ro`` register for backwards compatibility. Changed in 1.9: Either all qubits must be defined or all undefined. If qubits are undefined, this method will not help you. You must explicitly call `address_qubits` which will return a new Program. Changed in 1.9: This function now returns ``self`` and updates ``self._synthesized_instructions``. Changed in 2.0: This function will add an instruction to the top of the program to declare a register of bits called ``ro`` if and only if there are no other declarations in the program. :return: This object with the ``_synthesized_instructions`` member set. """

        self._synthesized_instructions = instantiate_labels(self._instructions)
        self._synthesized_instructions = implicitly_declare_ro(self._synthesized_instructions)
        return self