S:User-Startup:; BEGIN AsmPro ASSIGN AsmPro: <place where you put AsmPro binary> ; END AsmPro
If you include a file that uses BITDEF like exec/memory.i, you'll be unable to use \@ in macros.
Unique number generator used for \@ is a simple ctr++, and since the system includes are protected from multi-including (the usual #ifdef stuff), they are only evaluated in pass1. In pass2+, since all the values are defined, ctr++ is not executed the same number of times (it's reset to 0 at the start of every pass to ensure everything is in sync, but everything isn't in sync :P ) all those anonymous labels are consequently different and symbols are not found.