hs-set-up-overlay

hs-set-up-overlay is a variable defined in `hideshow.el'.
Its value is

nil

Documentation:
Function called with one arg, OV, a newly initialized overlay.
Hideshow puts a unique overlay on each range of text to be hidden
in the buffer. Here is a simple example of how to use this variable:

(defun display-code-line-counts (ov)
(when (eq 'code (overlay-get ov 'hs))
(overlay-put ov 'display
(format "... / %d"
(count-lines (overlay-start ov)
(overlay-end ov))))))

(setq hs-set-up-overlay 'display-code-line-counts)

This example shows how to get information from the overlay as well
as how to set its `display' property. See `hs-make-overlay' and
info node `(elisp)Overlays'.