presto-main/src/main/java/io/prestosql/sql/planner/SymbolsExtractor.java/SymbolsExtractor/visitSymbolReference
class SymbolsExtractor:        @Override
        protected Void visitSymbolReference(SymbolReference node, ImmutableList.Builder<Symbol> builder)
        {
            builder.add(Symbol.from(node));
            return null;
        }