presto-main/src/main/java/io/prestosql/sql/planner/sanity/NoIdentifierLeftChecker.java/NoIdentifierLeftChecker/validate
class NoIdentifierLeftChecker:    @Override
    public void validate(PlanNode plan, Session session, Metadata metadata, TypeAnalyzer typeAnalyzer, TypeProvider types, WarningCollector warningCollector)
    {
        List<Identifier> identifiers = ExpressionTreeUtils.extractExpressions(ExpressionExtractor.extractExpressions(plan), Identifier.class);
        if (!identifiers.isEmpty()) {
            throw new IllegalStateException("Unexpected identifier in logical plan: " + identifiers.get(0));
        }
    }