R4RS 6.6  (char-ci=? char1 char2)   ==>  boolean
          (char-ci<? char1 char2)   ==>  boolean
          (char-ci>? char1 char2)   ==>  boolean
          (char-ci<=? char1 char2)  ==>  boolean
          (char-ci>=? char1 char2)  ==>  boolean

These procedures are similar to CHAR=? et cetera, but they treat
upper case and lower case letters as the same. For example,

(char-ci=? #\A #\a)  ==>  #t.

Some implementations may generalize these procedures to take more
than two arguments, as with the corresponding numerical predicates.

The S9fES versions of these procedures do accept more than two
arguments.
