Function union

regex/regex-syntax/src/ast/parse.rs:4212–4217

(span: Span, items: Vec<ast::ClassSetItem>)

    #[test]
    fn parse_set_class() {
        fn union(span: Span, items: Vec<ast::ClassSetItem>) -> ast::ClassSet {
            ast::ClassSet::union(ast::ClassSetUnion {
                span: span,
                items: items,
            })
        }

        fn intersection(
            span: Span,