anchor.diff:    PersistentVector<E> set(int index, E value);
     PersistentVector<E> push(E value);
     PersistentVector<E> pop();
  -  TransientVector<E> asMutable();
  +  TransientVector<E> asTransient();
   
     factory PersistentVector() => new PersistentVectorImpl.empty();
     factory PersistentVector.from(Iterable<E> values) => new PersistentVectorImpl.from(values);
  
positive.code:     return new PersistentVectorImpl.empty();
    }
  
    TransientVectorImpl asMutable() => _asMutable();
    PersistentVectorImpl withMutations(fn) => _withMutations(fn);
    PersistentVectorImpl push(E value) => _push(value);
    PersistentVectorImpl pop() => _pop();
  
negative.code:     return this;
    }
  
    PersistentVectorImpl asPersistent() => _asImmutable();
    void doPush(E value) {
      _push(value);
    }
  