test-add-face-text-property.el

;; run it by command
;; $ emacs -Q --script test-add-face-text-property.el
(setq line "asdf qwer")
(add-face-text-property 0 4 'a t line)
(add-face-text-property 0 4 'a t line)
(print line)
;; #("asdf qwer" 0 4 (face a))

(add-face-text-property 0 5 'b t line)
(add-face-text-property 0 5 'b t line)
(add-face-text-property 0 5 'b t line)
(print line)
;; #("asdf qwer" 0 4 (face (a b b b)) 4 5 (face b))

(setq word (substring line 0 4))
(print word)
;; #("asdf" 0 4 (face (a b b b)))

(add-face-text-property 0 1 'c t word)
(add-face-text-property 0 2 'd t word)
(print word)
;; #("asdf" 0 1 (face (a b b b c d d)) 1 2 (face (a b b b c d d)) 2 4 (face (a b b b c d d)))

(print line)
;; #("asdf qwer" 0 4 (face (a b b b c d d)) 4 5 (face b))

Example for cpitclaudel/company-coq#252
Tested on emacs 26.3