a.lisp

(in-package #:hello)

(defparameter *a* "hello, ")

b.lisp

(in-package #:hello)

(defparameter *b* "World!")

hello.asd

(defpackage :hello-system (:use :asdf :cl))
(in-package :hello-system)

(defsystem hello
  :name "hello"
  :author ""
  :version ""
  :maintainer ""
  :licence ""
  :description ""
  :long-description ""
  :components
  ((:file "packages")
   (:file "a" :depends-on ("packages"))
   (:file "b" :depends-on ("packages"))
   ;; string-of-hello-world は packages.lisp、a.lisp、b.lispに依存している
   (:file "string-of-hello-world" :depends-on ("packages" "a" "b"))
   ;; body-of-hello-world.lisp は packages.lisp と string-of-hello-world.lisp に依存している
   ;; しかし、a.lisp と b.lisp への依存は明示しなくて良い
   (:file "body-of-hello-world" :depends-on ("packages" "string-of-hello-world"))))

string-of-hello-world.lisp

(in-package #:hello)

(defparameter *h* (concatenate 'string *a* *b*))