(defgeneric preprocess-command (shell-utility command &rest arguments)
  (:method ((s shell-utility) command &rest arguments)
    (declare (ignore arguments))
    (warn "Default method on PREPROCESS-COMMAND invoked; unknown command?")))

(defgeneric preprocess-commands (shell-utility)
  (:method ((shell-utility shell-utility))
    (dolist (shell-utility (commands shell-utility))
      (preprocess-command shell-utility command))))

(defmacro define-preprocess-command ((var class) (command &rest arguments) &body body)
  (with-gensyms (command-var args)
    `(defmethod preprocess-command ((,var ,class)
                                    (,command-var (eql ',command))
                                    &rest ,args)
       (destructuring-bind ,arguments ,args
         ,@body))))