For reusability, sometimes you might want to abstract out some custom reducer logic which can then be executed at arbitrary leaf state.
This can be done in two ways:
Shorthand 'riducers' are functions with the signature (leafState, action, treeState) => leafState.
When you pass a dictionary of these to riduce as a second argument, it automatically makes a corresponding action creator available.
By default, the action creator will take an optional single argument, that gets passed to your riducer logic as action.payload.
(This behaviour can be changed in a longhand riducer.)
Longhand riducer definitions benefit from being more strongly typed.