code: 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;
      }
docs: Removes a list of vertices whose 'head' is 'a' and whose 'tail' is b