anchor.diff:        (:error
          (unless (eql :input direction) (add-flags nix:o-excl)))
         (:error-if-symlink
  -       (add-flags nix:o-nofollow)
  -       (setf if-exists :overwrite)))
  +       (add-flags nix:o-nofollow)))
       (case if-does-not-exist
         (:create (add-flags nix:o-creat)))
       (cond
  
positive.code:                  :if-does-not-exist if-does-not-exist)))
  
  (defmethod device-open ((device file-device) &key filename flags mode
                          if-exists if-does-not-exist &allow-other-keys)
    (declare (ignore if-does-not-exist))
    (labels ((handle-error (c)
               (posix-file-error c filename "opening"))
  
negative.code:        (check-type if-exists (member nil :error-if-symlink))
         (check-type if-does-not-exist (member nil :error))
         (when (eql :default if-does-not-exist) (setf if-does-not-exist :error)))
        ((:output :io)
         (add-flags (if (eql :io direction) nix:o-rdwr nix:o-wronly))
         (check-type if-exists file-if-exists)
         (check-type if-does-not-exist file-if-does-not-exist)
         (when (eql :default if-exists) (setf if-exists :overwrite))
  