code: @Override
    public <N,V> Mutator<K> addInsertion(K key, String cf, HColumn<N,V> c) {
      getPendingMutations().addInsertion(key, Arrays.asList(cf),
          ((HColumnImpl<N, V>) c).toThrift());
      return this;
    }
docs: also, should throw a typed StatementValidationException or similar perhaps?