signature S = sig
        type t
        val x : t

        structure Q : sig
                type q
                val y : q
        end
end

structure S = struct
        type t = int
        val x = 0

        structure Q = struct
                type q = float
                val y = 0.0
        end
end

structure S1 = S
structure S2 : S = S
structure S3 = S2