presto-main/src/main/java/io/prestosql/sql/analyzer/FreeLambdaReferenceExtractor.java/FreeLambdaReferenceExtractor/visitIdentifier
class FreeLambdaReferenceExtractor:        @Override
        protected Void visitIdentifier(Identifier node, Set<String> lambdaArgumentNames)
        {
            if (analysis.getLambdaArgumentReferences().containsKey(NodeRef.of(node)) && !lambdaArgumentNames.contains(node.getValue())) {
                freeReferencesToLambdaArgument.add(node);
            }
            return null;
        }