* Introduce Result type to engine
The Result type can be used to signal the failure of a computation due
to both internal and non-internal reasons. If a computation failed due
to an internal error, the Result type carries that error with it and
provides it when the 'Error' method on a Result is called. If a
computation failed gracefully, but wished to bail instead of continue a
doomed plan, the 'Error' method provides a value of null.
* CR feedback
