presto-main/src/main/java/io/prestosql/spi/expression/ConnectorExpressionTranslator.java/ConnectorExpressionTranslator/visitSymbolReference
class ConnectorExpressionTranslator:        @Override
        protected Optional<ConnectorExpression> visitSymbolReference(SymbolReference node, Void context)
        {
            return Optional.of(new Variable(node.getName(), typeOf(node)));
        }