(fold f accum it)
  f: a callable
  accum: any type
  it: an iterable
  returns any type
F

Passes all of an iterator's items to an accumulator function.

Calls (= accum (f accum item)) for each item produced by the base iterator, then returns accum.

(prn (fold + 0 '(1 2 3 4 5))) ; prints 15
rfind
rfold