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Perhaps what I am about to say is obvious or has already been pointed out
(I haven't been paying much attention to this discussion), but David Gray's
note is related to the fact that the function type constructor is anti-
monotonic in its first argument, unlike most other other type constructors
which are monotonic in all arguments.  That is,

If      X is a subtype of Y
then    Z --> X is a subtype of Z --> Y
but     Y --> Z is a subtype of X --> Z.

It would be good if Common Lisp's notion of "type" and "subtype" could
be made consistent with this fact.

Peace, Will