make-overlay

make-overlay is a built-in function in `C source code'.

(make-overlay BEG END &optional BUFFER FRONT-ADVANCE REAR-ADVANCE)

Create a new overlay with range BEG to END in BUFFER and return it.
If omitted, BUFFER defaults to the current buffer.
BEG and END may be integers or markers.
The fourth arg FRONT-ADVANCE, if non-nil, makes the marker
for the front of the overlay advance when text is inserted there
(which means the text *is not* included in the overlay).
The fifth arg REAR-ADVANCE, if non-nil, makes the marker
for the rear of the overlay advance when text is inserted there
(which means the text *is* included in the overlay).