Seems perfectly reasonable to me, since streaming implies that there is not necessarily a notion of a beginning nor an end. Reducing has stronger requirements of its operations.

I will admit that the multiple arities is a bit awkward when an interface (or two) could have done the trick. I'm not 100% sure I understand why Rich choose to do it the way he did. I suspect it was so that `comp` would work.

I probably would have just defined ITransducer or something and supplied a custom composition function, but then again, the Haskell lens package takes the same approach, preferring composition via `.` on functions instead of extending a hypothetical "Composable" or "Pipelinable" type-class upon which `comp`/`.` could be built.