presto-main/src/main/java/io/prestosql/sql/analyzer/FreeLambdaReferenceExtractor.java/Visitor/visitLambdaExpression
class Visitor:        @Override
        protected Void visitLambdaExpression(LambdaExpression node, Set<String> lambdaArgumentNames)
        {
            return process(node.getBody(), ImmutableSet.<String>builder()
                    .addAll(lambdaArgumentNames)
                    .addAll(node.getArguments().stream()
                            .map(LambdaArgumentDeclaration::getName)
                            .map(Identifier::getValue)
                            .collect(toImmutableSet()))
                    .build());
        }