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Types in Scheme

    I'm sorry, can you tell me again why you'd want to ask the unportable
    ("intensional") question in portable code, or in any code, for that
    matter?  As someone trying to write good code, I want my program to be
    as insensitive as possible to the Scheme implementor's representation

I think you are being too "nitpicky" here.  Just because a piece of
code is not portable it does not mean it is not good.  To get some
efficiency (or detailed control of precision and accuracy), it may be
necessary, for example, to inquire whether a number (which might
otherwise be COMPLEX?, REAL?, RATNUM?, and INTEGER?) is a FIXNUM? in a
particular implementation.  I'm not advocating for
implementation-specific predicates in the language, but it is clear to
me that they are necessary for certain pieces of admittedly
non-portable code.  Each implementation should provide a set of
predicates about representation because it MAY matter to someone, and
s/he may have a valid use for them.