reduce should allow an accessor function to be specified

[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).


Mark Bromley
bromley@think.com