(assert (string= (string-downcase "This is a test.") "this is a test."))
(assert (string= (string-capitalize "This is a test.") "This Is A Test."))
(assert (string= (string-upcase "Is this 900-Sex-hott, please?" :start 3)
(assert (string= (string-downcase "This is a test.") "this is a test."))
(assert (string= (string-capitalize "This is a test.") "This Is A Test."))
(assert (string= (string-upcase "Is this 900-Sex-hott, please?" :start 3)