(apply #'format
       t
       "~%Test ~S is unknown; ~#[ no tests are known!~;~
test should be ~S~;~
test should be ~S or ~S~:;~
test should be one of ~@{~#[~1; or~] ~S~^,~}~]"
       'foo
       (mapcar #'car '((a) (b) (c))))

(format t "Test ~s is unknown~@[: should be~[~;~:; one of~] ~{~s~^, ~}~]" 'foo (length nil) nil)