(define double
  {number --> number}
  X -> (* X 2))
  
(define plus
  {number --> (number --> number)}
  0 X -> X
  X Y -> (+ 1 (plus (- X 1) Y)))

(define add1
  {(vector number) --> (vector number)}
   <> -> <> 
   (@v N V) -> (@v (+ N 1) (add1 V)))

(define whitespace*?
   {string --> boolean}
   S -> (let N (string->n S) (element? N [9 10 13  32])))

(define pi*
   {--> number}
    -> 3.142)
    
(define average 
          {number --> number --> number}
          M N -> (/ (+ M N) 2))