code: private void checkDualInputContract(DualInputOperator<?, ?, ?, ?> dualInputContract) {
  		Operator<?> input1 = dualInputContract.getFirstInput();
  		Operator<?> input2 = dualInputContract.getSecondInput();
  		// check if input exists
  		if (input1 == null || input2 == null) {
  			throw new MissingChildException();
  		}
  	}
docs: Checks whether a DualInputOperator is correctly connected. In case that
  the contract is incorrectly connected a RuntimeException is thrown.
  
  @param dualInputContract
  DualInputOperator that is checked.