+val interval_base = fn (EVar "access", _) => SOME "access"
+ | (EVar "modification", _) => SOME "modification"
+ | _ => NONE
+
+val interval = fn (EVar "years", _) => SOME "years"
+ | (EVar "months", _) => SOME "months"
+ | (EVar "weeks", _) => SOME "weeks"
+ | (EVar "days", _) => SOME "days"
+ | (EVar "hours", _) => SOME "hours"
+ | (EVar "minutes", _) => SOME "minutes"
+ | (EVar "seconds", _) => SOME "seconds"
+ | _ => NONE
+