Add Box.
Add Either.
Define map over Either.
Add ReducibleType.
The iteratees return Either.
reduce() over reducibles.
reduce() is public.
Left-reduction sans Either.
Add ReducibleOf<T>.
Simple left-reduction is public.
Add tests of ReducibleOf, demonstrating the crash.
Add either().
Add id().
Add a correct map(), returning Either.
Remove the incorrect map().
map() is a method.
Move id() into a Prelude file.

I couldn’t think of a better name.
Add fix().

h/t @CodaFi.
Use fix() to produce enumerators from reducibles.
Map the Optional.