anchor.diff:  
     /// The successor of this element in its linked list.
     ///
  -  /// The value is  `null` if there is no successor in the linked list,
  +  /// The value is `null` if there is no successor in the linked list,
     /// or if this entry is not currently in any list.
     E? get next {
       if (_list == null || identical(_list!.first, _next)) return null;
  
positive.code: ///
  /// A `LinkedList` also allows constant time adding and removing at either end,
  /// and a constant time length getter.
  class LinkedList<E extends LinkedListEntry<E>> extends Iterable<E> {
    int _modificationCount = 0;
    int _length = 0;
  
negative.code:     _insertBefore(_first, entry, updateFirst: false);
    }
  
    /// Adds [entries] to the end of the linked list.
    void addAll(Iterable<E> entries) {
      entries.forEach(add);
    }
  