can I do something like that ?

drawSomeThing(prameters..., void * screen) {
  graphlib_choose = 1;
  set "a1" = screen;
  .....
  graphlib_line(.....);
  .....
  graphlib_line(.....);
  .....
  graphlib_fill(....);
}

Or I'm obliged to do something like that ?

drawSomeThing(prameters..., void * screen) {
  graphlib_choose = 1
  .....
  set "a1" = screen;graphlib_line(.....);
  .....
  set "a1" = screen;graphlib_line(.....);
  .....
  set "a1" = screen;graphlib_fill(....);
}