allout-decorate-item-guides is a compiled Lisp function in `allout-widgets.el'.

(allout-decorate-item-guides ITEM-WIDGET &optional PARENT-WIDGET HAS-SUCCESSOR)

Add ITEM-WIDGET guide icon-prefix descender and connector text properties.

Optional arguments provide context for deriving the guides.
In their absence, the current guide column flags are used.

Optional PARENT-WIDGET is the widget for the item's parent item.

Optional HAS-SUCCESSOR is true if the item is followed by a sibling.

We also hide the header-prefix string.

Guides are established according to the item-widget's :guide-column-flags,
when different than :was-guide-column-flags. Changing that property and
reapplying this method will rectify the glyphs.