presto-main/src/main/java/io/prestosql/sql/planner/RelationPlanner.java/SetOperationPlan/SetOperationPlan
class SetOperationPlan:        private SetOperationPlan(List<PlanNode> sources, ListMultimap<Symbol, Symbol> symbolMapping)
        {
            this.sources = sources;
            this.symbolMapping = symbolMapping;
        }