presto-parser/src/main/java/io/prestosql/sql/SqlFormatter.java/SqlFormatter/visitUnion
class SqlFormatter:        @Override
        protected Void visitUnion(Union node, Integer indent)
        {
            Iterator<Relation> relations = node.getRelations().iterator();

            while (relations.hasNext()) {
                processRelation(relations.next(), indent);

                if (relations.hasNext()) {
                    builder.append("UNION ");
                    if (!node.isDistinct()) {
                        builder.append("ALL ");
                    }
                }
            }

            return null;
        }