Recognize applicability conditions.
(evmac-appcondp x) is a std::defaggregate of the following fields.
We represent an applicability condition as an aggregate.
The first field is a keyword that names the applicabiilty condition. This keyword should be the one shown in the user documentation of the event macro, and in progress or error messages on the screen when the event macro runs. The keyword may be one of a fixed finite set of possibilities, or could be calculated dynamically (e.g. to include a numeric index), based on the specifics of the event macro.
The second field is the formula (a term) that must be proved for the applicability condition to be satisfied.
More fields might be added in the future.