ANSI Common Lisp 22 Printer 22.2 The Lisp Pretty Printer 22.2.1 Pretty Printer Concepts
22.214.171.124 Compiling Format Strings
A format string is essentially a program in a special-purpose language
that performs printing, and that is interpreted by the function format.
The formatter macro provides the efficiency of using a compiled function
to do that same printing but without losing the textual compactness of format strings.
A format control is either a format string or a function
that was returned by the the formatter macro.