presto-main/src/main/java/io/prestosql/sql/planner/plan/EnforceSingleRowNode.java/EnforceSingleRowNode/EnforceSingleRowNode
class EnforceSingleRowNode:    @JsonCreator
    public EnforceSingleRowNode(
            @JsonProperty("id") PlanNodeId id,
            @JsonProperty("source") PlanNode source)
    {
        super(id);

        this.source = requireNonNull(source, "source is null");
    }