−The fold owns the traversal skeleton, leaving the calculation in a combining function that accepts just an accumulator and one element. That small function is easy to name and test; when several summary values are needed, a record accumulator can compute them in one pass.
+A fold handles the traversal while a combining function describes the calculation for an accumulator and one element. That small function is easy to name and test; when several summary values are needed, a record accumulator can compute them in one pass.
