anchor.diff:      _last = cons;
     }
   
  +  /**
  +   * Creates a new list prepending so far added elements to the
  +   * optional [tail].
  +   */
     LinkedList<E> build([tail = null]) {
       if (tail == null)
         tail = new Nil<E>();
  
positive.code:   LinkedList<E> strictWhere(bool f(A));
  }
  
  class LinkedListBuilder<E> {
    LinkedList<E> _first = null;
    Cons<E> _last = null;
  
    void add(E x) {
      Cons<E> cons = new Cons<E>(x, null);
      if (_first == null) {
  
negative.code:   bool moveNext() => false;
  }
  
  /**
   * Empty [LinkedList]
   */
  class Nil<E> extends _LinkedListBase<E> {
    bool get isNil => true;
    bool get isCons => false;
  