prefix-p.robin

;'<<SPEC'

    -> Tests for functionality "Evaluate Robin Expression (with List)"

`prefix?` evaluates its first and second arguments to obtain lists.
It then evaluates to `#t` if the first list is a prefix of the second
list, `#f` otherwise.  A list A is a prefix of a list B if A is `empty?`,
or the head of A is `equal?` to the head of B and the tail of A is a
prefix of the tail of B.

    | (prefix? (list 1 2 3) (list 1 2 3 4 5 6))
    = #t

    | (prefix? (list 1 2 5) (list 1 2 3 4 5 6))
    = #f

    | (prefix? () (list 1 2 3 4 5 6))
    = #t

    | (prefix? () (literal schpritz))
    = #t

    | (prefix? (list 1 2 3) (list 1 2 3))
    = #t

    | (prefix? (list 1 2 3 4) (list 1 2 3))
    = #f

'<<SPEC'

(define prefix? (fun (la lb)
  (bind prefix?-r (fun (self la lb)
    (if (empty? la)
      #t
      (if (empty? lb)
        #f
        (if (equal? (head la) (head lb))
          (self self (tail la) (tail lb))
          #f))))
    (prefix?-r prefix?-r la lb))))