Access the fifth constructor of a sum type.
Where Control.Lens._5 is the fifth projection out of a product, _I5 is the fifth
injection into a sum.
The default implementation is the prism to the fifth constructor of a
Generic type, injection (Proxy :: Proxy 4).
data T = C1 | C2 | C3 | C4 | C5 Int | C6 deriving (Show, Generic)instance Injection5 T T Int IntC5 1 ^? _I5Just 1
C6 ^? _I5Nothing
_I5 # 1 :: TC5 1