* gnus.el (gnus-overlay-get): New alias to overlay-get.

* gnus.el (gnus-overlay-get): New alias to overlay-get.
(gnus-overlays-in): New alias to overlays-in.

* gnus-sum.el (gnus-remove-overlays): Use gnus-overlays-in, gnus-overlay-get,
 and gnus-delete-overlay.
(gnus-summary-show-thread): Make it work as well for systems in which
 next-single-char-property-change is not available.
(gnus-summary-hide-thread): Use gnus-make-overlay and gnus-overlay-put.

* gnus-xmas.el (gnus-xmas-overlays-in): New function.
(gnus-overlay-get): New alias to extent-property.
(gnus-overlays-in): New alias to gnus-xmas-overlays-in.

* dgnushack.el: Autoload add-to-invisibility-spec for XEmacs 21.4 and SXEmacs.

* lpath.el: Fbind next-single-char-property-change for XEmacs 21.4 and SXEmacs.