def add_on_nr(key, to_add)
      result = req_list(new_req_list().add_add_on_nr(key, to_add))[0]
      process_result_add_on_nr(result)
    end
def add_on_nr(key, to_add)
      result = req_list(new_req_list().add_add_on_nr(key, to_add))[0]
      process_result_add_on_nr(result)
    end
Issues a add_on_nr operation to scalaris and adds it to the
 current transaction.
 Changes the number stored at the given key, i.e. adds some value.
 Assumes 0 if no value exists at key.