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(SYMBOL? #t) ==> ?

    Date: Fri, 11 Mar 88 19:10:23 PST
    From: Pavel.pa at Xerox.COM

    The type predicates are not useless in general, their use is simply
    unnecessary in *that* implementation of Scheme.  They are still
    necessary for the writing of portable code.

I must be missing something.  How could a predicate that might always
return true be at all useful in portable code?

Tell me how to write a symbolic differentiation program, or an
evaluator, that operates on the "obvious" s-expression inputs
(e.g. the list (/ 1 (sqrt (+ 1 x)))) in a world where the types number,
pair, symbol are not disjoint.