presto-main/src/main/java/io/prestosql/sql/planner/ExpressionExtractor.java/ExpressionExtractor/extractExpressionsNonRecursive
class ExpressionExtractor:    public static List<Expression> extractExpressionsNonRecursive(PlanNode plan)
    {
        ImmutableList.Builder<Expression> expressionsBuilder = ImmutableList.builder();
        plan.accept(new Visitor(expressionsBuilder::add, false, noLookup()), null);
        return expressionsBuilder.build();
    }