Flip list data structure for JavaScript. See docs.
const a = new FlipList( range(10000) );
const b = new FlipList( range(1, 20001) );
b.reverse();
a.extend(b);
a.reverse();
a.reversebetween(a.begin().next(), a.end().prev());
[...a]; 1, 1, 2, ..., 9999, 20000, ..., 3, 2, 0
