signature print_position =
sig
    type position
    val print = position -> ()
end

datatype position2D =
    Position2D of real * real;

datatype position3D =
    Position3D of real * real * real;

structure Pos2D : print_position =
struct
    type position = position2D
    val print pos = print("Position2D[" ^ Real.toString(#1 pos) ^ "," ^ Real.toString(#2 pos) ^ "]\n");
end

structure Pos3D : print_position =
struct
    type position = position3D
    val print pos = print("Position3D[" ^ Real.toString(#1 pos) ^ "," ^ Real.toString(#2 pos) ^ "," ^ Real.toString(#3 pos) ^ "]\n");
end