signature A = sig end
structure A = struct end
structure Ao : A = A


structure B = struct
        type t = int
end
structure Bo0 : sig end = B
structure BoA : A = B

signature B1 = sig
        type t
end
structure Bo1 : B1 = B
(*structure AoB1 : B1 = A*)

signature B2 = sig
        type t = int
end
structure Bo2 : B2 = B


structure C = struct
        type t = float
end
structure CoB1 : B1 = C
(*structure CoB2 : B2 = C*)