First method implementation: left?
Simplest two-class solution
Made left enumerable
Made Right enumerable
Can to_a
remove Enumerable.

If you want an enumerable, use to_a
transform for right
Left can left_transform
mapping the wrong side does nothing
Added flatmap by another name
Wait. I don't really need flatmap. Flattne.

Because flatMap, what, should it return an error if the call doesn't
return an Either? I think flatten should return that error. So
let's implement that.
implemented fold
Make it work as a gem