fun isEven(0) = true
  | isEven(1) = false 
  | isEven(n) = isEven(n-2);

fun isEven(0) = true
  | isEven(n) = not(isEven(n-1));

(* Ex 3.3.4, turnToInts *) 
fun turnToInts([]) = []
  | turnToInts(true::rest) = 1::turnToInts(rest)
  | turnToInts(false::rest) = 0::turnToInts(rest);

(* turnToBools *)
fun turnToBools([]) = []
  | turnToBools(0::rest) = false::turnToInts(rest) 
  | turnToBools(x::rest) = true::turnToInts(rest);



datatype bird = Robin | Sparrow | Crow | Cardinal;

fun containsRobin([]) = false
  | containsRobin(Robin::rest) = true
  | containsRobin(x::rest) = containsRobin(rest);