R4RS 6.6  (char-upcase char)    ==>  char
          (char-downcase char)  ==>  char

These procedures return a character CHAR2 such that

(char-ci=? char char2)  ==>  #t.

In addition, if CHAR is alphabetic, then the result of CHAR-UPCASE
is upper case and the result of CHAR-DOWNCASE is lower case.
