There is a Stata programming command or function that traps a user's specifying more than one of a set of mutually exclusive, collectively exhaustive options.

I believe that it used to be in the list of undocumented commands and functions, but I cannot find it anymore anywhere (help files & user manual entries for syntax, programming; Googling related phrases).

Can someone point me to it?

Typical use case would be like the following:
Code:
program myprogram
    syntax . . . ., [Up Down]

    _whatI'mlookingfor(up down)
If a user accidentally entered
Code:
myprogram . . ., up down
it would return an error to the effect that "only one of up down may be specified".