UNDEF <symbol>
The UNDEF directive cancels (undefines) a symbol created by a prior DEFINE, DEFALIAS or DEFTYPE directive.
For example:
++IFDEF Flavor_of_the_Month$
++MESSAGE Out with the old flavor (Flavor_of_the_Month$)
UNDEF Flavor_of_the_Month$
DEFINE Flavor_of_the_Month$ = "Marmite"
++ELSE
++MESSAGE Flavor of the month program not currently active
++ENDIF
Note that in the above case, without the UNDEF, the DEFINE statement would generate a "redefined symbol" error unless the new definition was the same as the prior one. On the other hand, an UNDEF of a symbol that hasn't yet been defined is not considered an error, so the above ++IFDEF would only be necessary or sensible if the objective was to change the flavor of the month only if a flavor of the month had already been established, and otherwise to leave it undefined.
History
2022 June, A-Shell 6.5.1717, compiler edit 994: UNDEF extended to work on DEFALIAS definitions. It previously worked only for DEFINEs.
