Removes a list of vertices whose 'head' is 'a' and whose 'tail' is b
function(a, b) {
      if (a.prev === null) {
        this.head = b.next;
      } else {
        a.prev.next = b.next;
      }

      if (b.next === null) {
        this.tail = a.prev;
      } else {
        b.next.prev = a.prev;
      }

      return this;
    }