Access the sixth constructor of a sum type.
Where Control.Lens._6 is the sixth projection out of a product, _I6 is the sixth
injection into a sum.
The default implementation is the prism to the sixth constructor of a
Generic type, injection (Proxy :: Proxy 5).
data T = C1 | C2 | C3 | C4 | C5 | C6 Int | C7 deriving (Show, Generic)instance Injection6 T T Int IntC6 1 ^? _I6Just 1
C7 ^? _I6Nothing
_I6 # 1 :: TC6 1