anchor.diff:    (:report
      (lambda (c s)
        (format s "Replacing existing command with name ~S"
  -             (redefining-command-name c)))))
  +             (redefining-command-name c))))
  +  (:documentation
  +   "A warning representing the fact that a shell command has been
  +replaced."))
   
   (defun install-command (name command &optional (priority (command-priority command)))
  +  "Install a command into the current command namespace.
  +
  +`name' is a string.  When the first element of a command invocation
  +matches the provided name, this command will be invoked.
  +
  +`command' is an object representing the command.  When it is time to
  +invoke the command, `invoke-command' will be called and the provided
  +command object will be passed in.
  +
  +`priority' is a number.  In the event that two commands have the same
  +`name', SHCL will invoke the command with the numerically smaller
  +priority."
     (when (find #\/ name)
       (warn 'confusing-command-name :name name))
     (let* ((table (command-namespace-table *command-namespace*))
  
positive.code: (defun register-special-builtin (name handler)
    (install-command name (make-instance 'special-builtin :handler handler)))
  
  (define-data builtin ()
    ((handler
      :initarg :handler
      :initform (required)
      :updater builtin-handler)))
  
  (defmethod documentation ((command builtin) (doc-type (eql t)))
    (documentation (builtin-handler command) doc-type))
  
negative.code:   (declare (ignore args))
    (error 'not-implemented :feature "exec"))
  
  (defgeneric exit-condition-exit-info (exit-condition)
    (:documentation
     "Retrieve the `exit-info' caried by an `exit-condition'."))
  
  (define-condition exit-condition (serious-condition)
    ((exit-info
      :initarg :exit-info
  