ANSI Common Lisp 13 Characters 13.1 Character Concepts 13.1.4 Character Categories
220.127.116.11 Characters With Case
The characters with case are
a subset of the alphabetic1 characters.
A character with case has the property of being either
uppercase or lowercase.
Every character with case is in one-to-one correspondence
with some other character with the opposite case.
18.104.22.168.1 Uppercase Characters
22.214.171.124.2 Lowercase Characters
126.96.36.199.3 Corresponding Characters in the Other Case
188.8.131.52.4 Case of Implementation-Defined Characters