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reduce should allow an accessor function to be specified

    Date: Sat, 14 Nov 87 04:38:32 est
    From: Mark Bromley <bromley@Think.COM>

    Currently, the function is directly applied to the elements of the sequence.
    This limits the utility of reduce.  Summing the cars of a sequence of conses is
    cumbersome using reduce, and is possible only because numbers can be
    distinguished from conses at run time.  E.g.

    (reduce #'(lambda (a b) (+ (if (consp a) (car a) a) (if (consp b) (car b) b))) sequence)

    It would be much more natural to be able to use the following

    (reduce #'+ sequence :key #'car).

A better interim solution, except for the fact that it conses more,
would be to use

	(reduce #'+ (map 'vector #'car sequence))

This doesn't depend on distinguishing previous results from input.