font-lock-fillin-text-property

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

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

Fill in one property of the text from START to END.
Arguments PROP and VALUE specify the property and value to put where none are
already in place. Therefore existing property values are not overwritten.
Optional argument OBJECT is the string or buffer containing the text.