presto-main/src/main/java/io/prestosql/operator/project/MergingPageOutput.java/MergingPageOutput/addInput
class MergingPageOutput:    public void addInput(Iterator<Optional<Page>> input)
    {
        requireNonNull(input, "input is null");
        checkState(!finishing, "output is in finishing state");
        checkState(currentInput == null, "currentInput is present");
        currentInput = input;
    }