open Array;

     (* checkAll(A,i) checks that array A has only true
        elements with indexes 0 through i *)

     fun checkAll(A,i) =
             i<0 orelse
                 sub(A,i) andalso checkAll(A,i-1);

     (* fillAndCheck(A,L) sets the element of array A to true
        for each letter appearing on list L, then checks
        that all elements are true *)

     fun fillAndCheck(A,nil) = checkAll(A,25)
     |   fillAndCheck(A,x::xs) = (
             update(A,ord(x)-ord(#"a"),true);
             fillAndCheck(A,xs)
     );

     fillAndCheck(array(26,false),
             explode("qwertyuiopasdfghjklzxcvbnm"));