Arguments and values to get.

[David C. Plummer <DCP@SCRC-QUABBIN.ARPA>]

Consider
	(incf (get foo bar))
when the property does not exist.  INCF would die adding to NIL.  Thus,
to maintain simple style, you need
	(incf (get foo bar 0))
I think this is a LOT prettier than
	(setf (get foo bar)
	      (multiple-value-bind (value valid-p)
		  (get foo bar)
		(if valid-p (1+ value) 1)))
and the programmer doesn't have to worry about multiple evaluation of
FOO and BAR, since INCF is supposed to take care of that.