code: public BulkMutation add(@Nonnull String rowKey, @Nonnull Mutation mutation) {
      Preconditions.checkNotNull(rowKey);
      Preconditions.checkNotNull(mutation);
  
      return add(ByteString.copyFromUtf8(rowKey), mutation);
    }
docs: Add mutation for a particular row. The changes in the mutation will be applied atomically.
  However there is no guarantees about the relative ordering between mutations affecting
  different rows.