anchor.diff:      return this;
     }
   
  -  PersistentVectorImpl asImmutable() => _asImmutable();
  +  PersistentVectorImpl asPersistent() => _asImmutable();
     void doPush(E value) {
       _push(value);
     }
  
positive.code:   void doSet(int index, E value);
    void doPush(E value);
    void doPop();
    PersistentVector<E> asImmutable();
  }
  
  abstract class PersistentVectorBase<E> extends IterableBase<E> {
  
negative.code:   PersistentVector<E> set(int index, E value);
    PersistentVector<E> push(E value);
    PersistentVector<E> pop();
    TransientVector<E> asTransient();
  
    factory PersistentVector() => new PersistentVectorImpl.empty();
    factory PersistentVector.from(Iterable<E> values) => new PersistentVectorImpl.from(values);
  