Info-insert-dir

[martin rudalics]

I fail to understand the following code in `Info-insert-dir':

    (let ((dirs (if Info-additional-directory-list
                    (append Info-directory-list
                            Info-additional-directory-list)
                  Info-directory-list))
      ...
      ;; Search the directory list for the directory file.
      (while dirs
          ....
          (unless (cdr dirs)
            (set (make-local-variable 'Info-dir-contents-directory)
                 (file-name-as-directory (car dirs))))
          (setq dirs (cdr dirs))))

According to the doc-string of `Info-directory-list'

"the directory of Info files that come with Emacs
is put last (so that local Info files override standard ones)"

Apparently that's what the "unless ..." form relies upon to produce a
default-directory.  When `Info-additional-directory-list' is non-nil the
"append ..." form appends that list and the "unless ..." form will
return the last directory in `Info-additional-directory-list'.  If that
directory does not exist, the value of default-directory is nil.