presto-main/src/main/java/io/prestosql/sql/planner/optimizations/PruneUnreferencedOutputs.java/PruneUnreferencedOutputs/rewriteSetOperationSymbolMapping
class PruneUnreferencedOutputs:        private ListMultimap<Symbol, Symbol> rewriteSetOperationSymbolMapping(SetOperationNode node, RewriteContext<Set<Symbol>> context)
        {
            // Find out which output symbols we need to keep
            ImmutableListMultimap.Builder<Symbol, Symbol> rewrittenSymbolMappingBuilder = ImmutableListMultimap.builder();
            for (Symbol symbol : node.getOutputSymbols()) {
                if (context.get().contains(symbol)) {
                    rewrittenSymbolMappingBuilder.putAll(symbol, node.getSymbolMapping().get(symbol));
                }
            }
            return rewrittenSymbolMappingBuilder.build();
        }