presto-main/src/main/java/io/prestosql/operator/GroupedTopNBuilder.java/GroupedTopNBuilder/dereference
class GroupedTopNBuilder:        public void dereference(int position)
        {
            checkArgument(reference[position] != null && usedPositionCount > 0);
            reference[position] = null;
            usedPositionCount--;
        }