anchor.diff:      return new PersistentVectorImpl.empty();
     }
   
  -  TransientVectorImpl asMutable() => _asMutable();
  +  TransientVectorImpl asTransient() => _asMutable();
     PersistentVectorImpl withMutations(fn) => _withMutations(fn);
     PersistentVectorImpl push(E value) => _push(value);
     PersistentVectorImpl pop() => _pop();
  
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);
  