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    don't do anything related to relocating, just return nil.
  (bookmark-error-no-filename): New error.
  (bookmark-default-handler): Signal `bookmark-error-no-filename' if
    bookmark has no file.  Don't even attempt to handle things that
    are not files; the whole point of custom handlers is to keep that
    knowledge elsewhere anyway.  Tighten some comments.
  (bookmark-file-or-variation-thereof): Remove now-unused function.
  (bookmark-location): Doc string fix.

(Bug#4250)