turn upper-case characters into lower-case characters
Major Section:  ACL2-BUILT-INS

(Char-downcase x) is equal to #\a when x is #\A, #\b when x is #\B, ..., and #\z when x is #\Z, and is x for any other character.

The guard for char-downcase requires its argument to be a standard character (see standard-char-p).

Char-downcase is a Common Lisp function. See any Common Lisp documentation for more information.

To see the ACL2 definition of this function, see pf.