code: public boolean add(T item) {
          ListNode<T> node = new ListNode<>(item);
          synchronized (this.lock) {
              if (this.tail == null) {
                  // List is currently empty.
                  this.head = node;
              } else {
                  if (item.getSequenceNumber() <= this.tail.item.getSequenceNumber()) {
                      // Item to be added is not in order - reject it.
                      return false;
                  }
  
                  this.tail.next = node;
              }
  
              this.tail = node;
          }
  
          return true;
      }
docs: Adds a new item at the end of the list, but only if the given item has a Sequence Number higher than the last
  element in the list.
  
  @param item The item to append.
  @return True if the item was added (meets sequencing criteria or list was empty), false otherwise.