code: void linkBefore(E e, Node<E> succ) {
          // assert succ != null;
          final Node<E> pred = succ.prev;
          final Node<E> newNode = new Node<>(pred, e, succ);
          succ.prev = newNode;
          if (pred == null)
              first = newNode;
          else
              pred.next = newNode;
          size++;
          modCount++;
      }
docs: Inserts element e before non-null Node succ.