The problems was a code 18: 18 inches in front of the monitor!

The expansion works fine as expected.

I was using this structure:

        _do

                 ;do stuff

        _while (cond)

The correct macro in this case would be:

        _do

                 ;do stuff

        _do_while (cond)

or

        _while (cond)

                 ;do stuff

        _while_end