anchor.diff:    bool get isEmpty => _left == null;
   
     @override
  -  Iterator<E> get iterator => new _ImmutableListIterator(this);
  +  Iterator<E> get iterator => new _AvlImmutableListIterator(this);
   
     @override
     _AvlNode get left => _left;
  
positive.code:   }
  
    Iterator<E> _iterator(ImmutableListBuilder builder) =>
        new _ImmutableListIterator(this, builder: builder);
  
    /// Creates a node mutation, either by mutating this node (if not yet frozen)
    /// or by creating a clone of this node with the described changes.
  
negative.code: 
    @override
    Iterator<E> get iterator =>
        new _AvlImmutableListIterator<E>(_root, reversed: true);
  }
  
  class _SubListIterable<E> extends IterableBase<E> {
  