Takes two required arguments:
- destination for output
- ‘t’ : the ‘*standard-output*’ stream;
- ‘nil’ :
format generates a string and returns it;
- a stream : output written to this stream;
- string with fill-pointer : ougtput is added to the end of the string and
the fill pointer is adjusted;
- control-string that contains literal text and
embedded directives;
- additional optional format-args provide the
values used by the directives in the control string that interpolate values
into the output.
format returns ‘nil’ except when destination is ‘nil’, in which case it
returns the newly-generated string.