(defpackage "YOUR-SCHEME-IMPLEMENTATION"
  (:use "COMMON-LISP")
  (:documentation "Implements a scheme in Common-Lisp")
  (:export "DEFINE" "RUN"))

(defpackage "YOUR-SCHEME-USER"
  (:use "YOUR-SCHEME-IMPLEMENTATION")
  (:documentation "The run-time your-scheme user package."))

(in-package "YOUR-SCHEME-IMPLEMENTATION") ; <--

(defun run ()
  (let ((*package* (or (find-package "YOUR-SCHEME-USER") ; <--
                       (error "Cannot find runtime scheme user package."))))
    (loop
      :named REPL
      :for form := (read)
      :for result := (scheme-eval form)
      :do (print result))))