Index lens.

>>> ('a' L.::: 'b' L.::: 'c' L.::: L.VNil) ^. L._Pull . ix (FS FZ)
'b'

>>> ('a' L.::: 'b' L.::: 'c' L.::: L.VNil) & L._Pull . ix (FS FZ) .~ 'x'
'a' ::: 'x' ::: 'c' ::: VNil

Match on non-empty Vec.

Note: lens _Cons is a Prism. In fact, Vec n a cannot have an instance of Cons as types don't match.

Head lens. Note: lens _head is a Traversal'.

>>> ('a' L.::: 'b' L.::: 'c' L.::: L.VNil) ^. L._Pull . _head
'a'

>>> ('a' L.::: 'b' L.::: 'c' L.::: L.VNil) & L._Pull . _head .~ 'x'
'x' ::: 'b' ::: 'c' ::: VNil

Head lens. Note: lens _head is a Traversal'.

Prism from list.