code: def _synthesize(self):
          
          self._synthesized_instructions = instantiate_labels(self._instructions)
          self._synthesized_instructions = implicitly_declare_ro(self._synthesized_instructions)
          return self
docs: 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.