Structural subtype relationships between sum types.

Running example:

>>> :set -XTypeApplications
>>> :set -XDataKinds
>>> :set -XDeriveGeneric
>>> import GHC.Generics
>>> import Control.Lens
>>> :{
data Animal
  = Dog Dog
  | Cat Name Age
  | Duck Age
  deriving (Generic, Show)
data FourLeggedAnimal
  = Dog4 Dog
  | Cat4 Name Age
  deriving (Generic, Show)
data Dog = MkDog
  { name :: Name
  , age  :: Age
  }
  deriving (Generic, Show)
type Name = String
type Age  = Int
dog, cat, duck :: Animal
dog = Dog (MkDog "Shep" 3)
cat = Cat "Mog" 5
duck = Duck 2
dog4, cat4 :: FourLeggedAnimal
dog4 = Dog4 (MkDog "Snowy" 4)
cat4 = Cat4 "Garfield" 6
:}