A SinkTIn, TMat that will invoke the given reduce for every received element, giving it its previous output (from the second element) and the element as input. The returned TaskTResult will be completed with value of the final function evaluation when the input stream ends, or completed with `Failure` if there is a failure signaled in the stream. If the stream is empty (i.e. completes before signaling any elements), the sum stage will fail its downstream with a NoSuchElementException, which is semantically in-line with that standard library collections do in such situations.

public static Sink<TIn, Task<TIn>> Sum<TIn>(
	Func<TIn, TIn, TIn> reduce
)

static member Sum : 
        reduce : Func<'TIn, 'TIn, 'TIn> -> Sink<'TIn, Task<'TIn>>