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 ANSI Common Lisp    22 Printer    22.2 The Lisp Pretty Printer    22.2.1 Pretty Printer Concepts
22.2.1.3   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.
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