char string index	character
schar string index	character

(setf (char string index) new-character)
(setf (schar string index) new-character)

index - a valid array index for the string.

character, new-character - a character.

char ignores fill pointers when accessing elements.

 (setq my-simple-string (make-string 6 :initial-element #\A))  "AAAAAA"
 (schar my-simple-string 4)  #\A
 (setf (schar my-simple-string 4) #\B)  #\B
 my-simple-string  "AAAABA"
 (setq my-filled-string
       (make-array 6 :element-type 'character
                     :fill-pointer 5
                     :initial-contents my-simple-string))
 "AAAAB"
 (char my-filled-string 4)  #\B
 (char my-filled-string 5)  #\A
 (setf (char my-filled-string 3) #\C)  #\C
 (setf (char my-filled-string 5) #\D)  #\D
 (setf (fill-pointer my-filled-string) 6)  6
 my-filled-string  "AAACBD"

 (char s j) ==(aref (the string s) j)