Use special macro variables in special macro functions

[MareXtafl]

Hello.
I'm trying to save actual XY coordinates to user variable for later use:

{default_double(tmp_pos_x,((bounding_box[3]-bounding_box[0])/2+bounding_box[0]+current_position[0])}          
{default_double(tmp_pos_y,((bounding_box[4]-bounding_box[1])/2+bounding_box[1]+current_position[1])}

But it doesn't work:

pause_print_gcode
Parsing error at line 7. Expecting tag real
{default_double(tmp_pos_x,((bounding_box[3]-bounding_box[0])/2+bounding_box[0]+current_position[0])}
                          ^

I tried to use a direct number {default_double(tmp_pos_x, -10.452)} and it works but when I use special macro variable {default_double(tmp_pos_x, current_position[0])} it results "Expecting tag real". I don't know what means "tag real".
I tried:
{default_double(tmp_pos_x, {current_position[0]})}
{default_double(tmp_pos_x, [current_position[0]])}
{default_double(tmp_pos_x, (current_position[0]))}
All of them result the same error.

Is it possible to use macro variables in macro functions and if so how?

Thank you and sorry for my bad English.