gistfile1.txt

Source:
(: f (Number [#:y Boolean] -> Number))
(define (f x #:y [y #f] #:z [z 'this-can-be-anything])
  (if y "y is truthy" x))
  
Expanded inner function:
'(lambda (y1 y3 z2 z4 x5)
   (let-values (((x) x5))
     (let-values (((y) (if y3 y1 '#f)))
       (let-values (((z) (if z4 z2 'this-can-be-anything)))
         (let-values () (if y '"y is truthy" x))))))
         
Type we use to check it:
(case-> (-> Boolean True Nothing True Number Number) (-> False False False False Number Number))