#!/usr/local/bin/sbcl --script

(defmacro -> (form . forms)
  (if (null forms)
      form
    `(-> (nsubseq ,form it ,(car forms)) ,@(cdr forms))))

(defun join-string (lst sep)
  (-> (reduce (lambda (acc s)
                (list* s "a" acc))
              (cdr x)
              :initial-value (list (car x)))
      (nreverse it)
      (apply #'concatenate 'string it)))

(let ((x (list "a" "b" "c" "d")))
  (print (join-string x " ")))