The readtable case of the current readtable affects the Lisp reader in the following ways:
:upcase
When the readtable case is :upcase
,
unescaped constituent characters are converted to uppercase,
as specified in Reader Algorithm.
:downcase
When the readtable case is :downcase
,
unescaped constituent characters are converted to lowercase.
:preserve
When the readtable case is :preserve
,
the case of all characters remains unchanged.
:invert
When the readtable case is :invert
,
then if all of the unescaped letters in the extended token are of the same case,
those (unescaped) letters are converted to the opposite case.