Equirecursive types

type 'a list = [ `Nil | `Cons of 'a * 'a list ]

let twice : 'a -> [ `Nil | `Cons of 'a * [ `Nil | `Cons of 'a * 'a list ] ] =
    fun x -> `Cons (x, `Cons (x, `Nil))

let xs : int list =
    twice 1