co-completeness checking

It would be really neat to also be able to optionally invoke completeness checking in the dual situation to the one where one typically does.

For example, when writing a parser you often want to make sure you’ve implemented a parser for each case of a coproducts / variants / enums. It would be neat if one could automatically derive a dual type operator and use completeness checking to ensure that each variant is covered.

Strictly speaking, perhaps co-completeness checking is what I call completeness checking for records, and the actual desire expressed by this note is the ability to explicitly represent the dual of a coproducts / variants / enums. Perhaps the dual type could explicitly be used as part of the implementation of pattern matching.