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#f = #none ?

Imagine a function which takes an optional argument which is to be
True, False, or Dont-care.  Presumably if the argument is omitted, the
value should be Dont-care.  How will we encode these values?  We can't
use #f for False, because it also means that the argument isn't there.
We could use #t for true, but we'll have to be careful, because all
boolean functions in Dylan allow any non-#f value to represent

So perhaps we'll need to define a new type which takes values
(singletons) True, False, and #f...  Seems silly, and error-prone.