~% Directive

["Scott E. Fahlman" <Fahlman@C.CS.CMU.EDU>]

    do you know what this should do?

    (FORMAT NIL "~-1%")

Two choices:

1. "It is an error."
2. "Crank up the monitor's voltage and send a lethal blast of X-rays
   into the face of the person issuing such a silly command."

Since not all machines can support solution 2, we should probably go
with 1, even though 2 is the more elegant and long-lasting solution.

But seriously, there seem to be a number of places in the format
directives where negative numbers would make no sense.  I don't think
that all of these are explicitly flagged.  This should probably be fixed
up in the standard, but until then it seems reasonable to assume
non-negative integers unless there's some obvious meaning for the
negative case.

-- Scott