[dminik]

I find these two statements to be contradictory

> Sure, but that's abstracted away from you

> Mixing futures from any two different `Io` implementations will typically result in Illegal Behavior

Thinking about it more, you've possibly added even more colors. Each executor adds a different color and while each function is color-agnostic (but not colorless) futures aren't.

> it will be extraordinarily rare to have more than one `Io`

Will it? I can immediately think of a use case where a program might want to block for files on disk, but defer fetching from network to some background async executor.