def add_on_nr(key, to_add)
      result = @conn.call(:add_on_nr, [key, to_add])
      @conn.class.check_fail_abort(result)
      @conn.class.process_result_add_on_nr(result)
    end
def add_on_nr(key, to_add)
      result = @conn.call(:add_on_nr, [key, to_add])
      @conn.class.check_fail_abort(result)
      @conn.class.process_result_add_on_nr(result)
    end
Changes the number stored at the given key, i.e. adds some value.
 Assumes 0 if no value exists at key.