dispatching on keywords

[Stavros Macrakis <macrakis@osf.org>]

   The problem is really in the assumption that keyword arguments that
   aren't provided by the user must be assigned a default value.  I
   suggest that: "If it hurts, don't do that."  It is possible to
   leave such arguments "unassigned" and leave it to the programmer to
   detect and assign a default, if desired, _after_ method dispatch.

Agreed.  Yet another role for #none (or #nothing).  For that matter,
this gives the programmer a way of explicitly saying "I know that
there is a keyword K, but I do not want to provide a value for it",
not to be confused with "...and I give it the value #f":

		K: #none

	-s