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char  string index ⇒  character
schar  string index ⇒  character
(setf (char string index) new-character)
(setf (schar string index) new-character)
string—for char, a string; for schar, a simple string.
index—a valid array index for the string.
character, new-character—a character.
char and schar access the element of string specified by index.
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)