Reduce in Lisp

The function reduce applies a specified function to the first two elements of a list, then to the result of the first two and the third element, and so forth.

>(reduce '+ '(1 2 3 17))

23

>(reduce '* '(1 2 3 17))

102

reduce is what we need to process a result from mapcan:

>(reduce '+ (mapcan 'testforz
                    '(z m u l e z r u l e z)))
                     (1)       (1)       (1)
= (reduce '+ '(1 1 1))

3