Utilities that avoid repetition when declaring instances for types that are not amenable to deriving, and in particular, types with existential quantification.

For existentials that only quantify one type variable, this module's functionality is mostly superseded by the some Hackage package. However, this library involves many existentials that quantify multiple variables. That can be shoehorned into some with some encoding, but I believe this module's weight is preferable to the overhead of using that encoding in our existential data types' declarations.