Formatted Output to Character Streams

CLtL2 Reference 

function FORMAT : (destination control-string &rest arguments)
Format ARGUMENTS to the text stream DESTINATION according to the specifications in CONTROL-STRING. SubL format is more limited than the Common Lisp version: the only escapes allowed in the control string are: ~A ~a ~S ~s ~D ~d (with width and pad parameters optional) ~C ~c ~G ~g ~% ~~