docs: def noisy_moment(self, moment: 'cirq.Moment',
                       system_qubits: Sequence['cirq.Qid']) -> 'cirq.OP_TREE':
          """Adds noise to the operations from a moment.
  
          Args:
              moment: The moment to add noise to.
              system_qubits: A list of all qubits in the system.
  
          Returns:
              An OP_TREE corresponding to the noisy operations for the moment.
          """
          if not hasattr(self.noisy_moments, '_not_overridden'):
              return self.noisy_moments([moment], system_qubits)
  
          if not hasattr(self.noisy_operation, '_not_overridden'):
              return [self.noisy_operation(op) for op in moment]
  
          assert False, 'Should be unreachable.'