reduce

(reduce function sequence &key initial-value from-end key start end)

Combines the elements of sequence with a binary function, left-associatively: (reduce #'f '(a b c)) computes (f (f a b) c). The sequence may be a list or a string, whose characters are folded. With :initial-value the seed is supplied explicitly and folded in first: (f (f (f init a) b) c); otherwise the first element of the sequence is the seed. With :from-end t the elements are combined right-associatively with the accumulator on the right: (reduce #'f '(a b c) :from-end t) computes (f a (f b c)), and with :initial-value i, (f a (f b (f c i))). :key names a one-argument function applied to each sequence element (but not the initial value) before folding. :start/:end restrict the fold to a subsequence (an :end of nil means the whole sequence). Keywords may appear in any order; the keyword names are read at compile time (their values may be runtime expressions). An empty sequence returns the initial value, or calls function with no arguments when none was given.