To the best of my knowledge, `InstanceSigs` are never strictly necessary.
They can, however, be useful for at least four purposes:
1. To provide a compiler-checked reminder of the type.
2. To bind type variables with `ScopedTypeVariables`.
3. To generalize the type so you can use polymorphic
I can't help but feel InstanceSigs are either superfluous or upside-down.
It's this bit in the User Guide:
> The type signature in the instance declaration must be
> more polymorphic than (or the same as) the one in the class declaration,
> instantiated with the instance type.
Usually if you
