ANSI Common Lisp 22 Printer 22.3 Formatted Output
22.3.12 Notes about FORMAT
Formatted output is performed not only by format,
but by certain other functions that accept a format control
the way format does. For example, error-signaling functions
such as cerror accept format controls.
Note that the meaning of nil and t as destinations to format
are different than those of nil and t as stream designators.
The ~^ should appear only at the beginning of a ~< clause,
because it aborts the entire clause in which it appears (as well as
all following clauses).