font-lock-append-text-property

font-lock-append-text-property is a compiled Lisp function in `font-lock.el'.

(font-lock-append-text-property START END PROP VALUE &optional OBJECT)

Append to one property of the text from START to END.
Arguments PROP and VALUE specify the property and value to append to the value
already in place. The resulting property values are always lists.
Optional argument OBJECT is the string or buffer containing the text.