anchor.diff:    LinkedList<E> strictWhere(bool f(A));
   }
   
  +/**
  + * [LinkedList] builder.
  + *
  + * Elements are added from the first.
  + */
   class LinkedListBuilder<E> {
     LinkedList<E> _first = null;
     Cons<E> _last = null;
   
  +  /// Adds the next element to the list
     void add(E x) {
       Cons<E> cons = new Cons<E>(x, null);
       if (_first == null) {
  
positive.code:     _last = cons;
    }
  
    LinkedList<E> build([tail = null]) {
      if (tail == null)
        tail = new Nil<E>();
  
negative.code:   }
  }
  
  /**
   * Nonempty [LinkedList]
   */
  class Cons<E> extends _LinkedListBase<E> {
    int _length = null;
  
    /// The first element of this
    final E elem;
  
    /// The rest of this - without the first element
    LinkedList<E> tail;
  
    Cons(this.elem, this.tail);
  