presto-main/src/main/java/io/prestosql/sql/planner/sanity/ValidateDependenciesChecker.java/ValidateDependenciesChecker/visitSetOperation
class ValidateDependenciesChecker:        private Void visitSetOperation(SetOperationNode node, Set<Symbol> boundSymbols)
        {
            for (int i = 0; i < node.getSources().size(); i++) {
                PlanNode subplan = node.getSources().get(i);
                checkDependencies(subplan.getOutputSymbols(), node.sourceOutputLayout(i), "%s subplan must provide all of the necessary symbols", node.getClass().getSimpleName());
                subplan.accept(this, boundSymbols); // visit child
            }

            return null;
        }