Fractran executor

gistfile1.txt

(defun multiple (a b)
  "Is A a multiple of B?"
  (= (mod a b) 0))

(defun next-fraction (n fractions)
  "Returns the next value of F in the Fractran program."
  (find-if (lambda (f)
             (integerp (* n f)))
           fractions))

(defun print-fractran-alphabet (n chars)
  "After moving to state N, print all of the necessary characters."
  (loop for (char . number) in chars
        when (multiple n number)
          do (princ char)))

(defun run-fractran (fractions alphabet &optional (n 2))
  "Run the given Fractran program. ALPHABET is an alist of characters to numbers
   representing the alphabet."
  (let ((f (next-fraction n fractions)))
    (if (null f)
        nil
        (let ((next (* n f)))
          (print-fractran-alphabet next alphabet)
          (run-fractran fractions alphabet next)))))