(define (f g)
  (g 2))

(f square)
; 4

(f (lambda (z) (* z (+ z  1))))
; 6

; 1 ]=> (f f)
; 
; ;The object 2 is not applicable.
; 
; (f f)
; (f 2)
; (2 2)
; => error 2 is not applicable