S9fES  (fold-left procedure object list ...)  ==>  object

Fold a series of lists by combining the result so far with adjacent
members of the given LISTs by PROCEDURE. PROCEDURE must by a K-ary
procedure where K is the number of lists passed to FOLD-LEFT plus
one. OBJECT is the base element that is used in the place of the
intermediate result in the first application of PROCEDURE. FOLD-LEFT
folds lists by grouping applications of PROCEDURE to the left.

Applications of FOLD-LEFT can be rewritten as follows:

(fold-left p2 0 (a b))              ==  (p2 (p2 0 a) b)
(fold-left p3 0 (a b) (c d))        ==  (p3 (p3 0 a c) b d)
(fold-left p4 0 (a b) (c d) (e f))  ==  (p4 (p4 0 a c e) b d f)

(fold-left cons 0 '(1 2 3))              ==>  (((0 . 1) . 2) . 3)
(fold-left list 0 '(1 2) '(3 4) '(5 6))  ==>  ((0 1 3 5) 2 4 6)
(fold-left + 0 '(1 2 3))                 ==>  6
(fold-left - 0 '(1 2 3))                 ==>  -6
