The DefaultValue for SET parameters may be a default set of values, the keyword ALL to indicate all possible values for that type of set, or empty square brackets ([ ]) to indicate no possible value for that type of set (and empty set).

SET OF INTEGER1 SetValues := [5,10,15,20];

IsInSetFunction(SET OF INTEGER1 x=SetValues,y) := y IN x;

OUTPUT(IsInSetFunction([1,2,3,4],5)); //false
OUTPUT(IsInSetFunction(,5)); // true