REDUCE-TREE

Applies the commutative binary <function> recursively throughout the list <self>.
(Applies to the first elements, then the result to the next one, and so forth until the list is exhausted.)

Function '+, for instance, makes reduce-tree computing the sum of all elements in the list.

Optional <accum> should be the neutral element for the <function> considered (i.e. initial result value).
If <accum> is nil, figures out what the neutral can be (works for +,*,min,max).