fold

Performs a reduction operation on the elements of this seq. This is equivalent to:

A result = identity;
foreach (G g in seq) {
    result = accumulator(g, result);
}
return result;

but is not constrained to execute sequentially.

The identity value must be an identity for the combiner function. it means that: a is equal to combiner(identity, a)

The combiner function must be compatible with the accumulator function. it means that: accumulator(g, a) is equal to combiner(accumulator(identity, g), a)

This is a terminal operation.

Parameters:

accumulator	an associative, non-interfering, and stateless function for accumulating
combiner	an associative, non-interfering, and stateless function for combining two values
identity	the identity value for the combiner function

Returns:

a future of the result of the reduction