(define smallest-in-list
  (lambda (numbers)
    (if (not (list? numbers))
        (error 'smallest-in-list "expects a list and not whatever type you entered")
        (if (null? numbers)
            (error 'smallest-in-list "expects a non-empty list")
            (if (not (all-real-numbers? numbers))
                (error 'smallest-in-list "expects a list of real numbers")
                (smallest-in-list-kernel numbers))))))

(define smallest-in-list-kernel
  (lambda (numbers)
    ; If there's only one element,
    (if (null? (cdr numbers))
        ; It must be the smallest element
        (car numbers)
        ; Otherwise, take the smaller of the first and the smallest
        ; remaining element.
        (min (car numbers) (smallest-in-list-kernel (cdr numbers))))))

; Determine if all the numbers in a list are real
(define all-real-numbers?
  (lambda (lst)
    (display "Checking ")
    (display lst)
    (newline)
    (or (null? lst)
        (and (number? (car lst))
             (real? (car lst))
             (all-real-numbers? (cdr lst))))))
  
; Husk and kernel programming: The husk checks the preconditions.  The kernel
; does the work.