code: private void checkSingleInputContract(SingleInputOperator<?, ?, ?> singleInputContract) {
  		Operator<?> input = singleInputContract.getInput();
  		// check if input exists
  		if (input == null) {
  			throw new MissingChildException();
  		}
  	}
docs: Checks whether a SingleInputOperator is correctly connected. In case that
  the contract is incorrectly connected a RuntimeException is thrown.
  
  @param singleInputContract
  SingleInputOperator that is checked.