removeChain (a, b) {
    if (!a.prev) {
      this.head = b.next
    } else {
      a.prev.next = b.next
    }

    if (!b.next) {
      this.tail = a.prev
    } else {
      b.next.prev = a.prev
    }
  }
Removes a chain of nodes whose head is `a` and whose tail is `b`,
it's assumed that `a` and `b` belong to this list and also that `a`
comes before `b` in the linked list

@param {*} a
@param {*} b