Shortcuts for choosing specific outputs

[markrwilliams]

Per @moshez's suggestion:

The collector callable passed to upon runs on a generator of outputs. That means replacing it with next, as suggested by the docs, for a transition that includes multiple outputs will result in only the first output running!

Often times you just want the last output's result. By adding a choose argument to upon, a user can use a numeric index to extract their desired output. For example, this:

someState.upon(someInput, 
               enter=anotherState,
               outputs=[firstOutput, secondOutput],
               choose=-1)

...would mean that someMachine.someInput() would run both firstOutput and secondOutput, but only return secondOutput's result.

What about choosing a specific output instead? Like so:

someState.upon(someInput, 
               enter=anotherState,
               outputs=[firstOutput, secondOutput],
               choose=secondOutput)

Is that too verbose?