anchor.diff:      Result = do_update({Props}, Update),
       case has_operators(Result) of
           true ->
  -            throw(update_leaves_operators);
  +            ?MANGO_ERROR(update_leaves_operators);
           false ->
               ok
       end,
  
positive.code:     Props;
  do_update_inc(Props, [{Field, Incr} | Rest]) ->
      if is_number(Incr) -> ok; true ->
          throw({invalid_increment, Incr})
      end,
      NewProps = case get_field(Props, Field, fun is_number/1) of
          Value when is_number(Value) ->
  
negative.code:                 {ValueProps} ->
                      Fun(Props, ValueProps);
                  _ ->
                      ?MANGO_ERROR({invalid_operand, Op, Value})
              end
      end,
      do_update(NewProps, Rest).
  