object GenericObj =  gs:GenericData;

object SpecificObj1 extends GenericObj =  ss1:SpecificData1;
object SpecificObj2  extends GenericObj =  ss2:SpecificData2;
object SpecificObj1a extends SpecificObj1 =  ss1a:SpecificData1;

object GenericData =  integer;
object SpecificData1 =  integer;
object SpecificData2 =  integer;
object SpecificData1a =  integer;

operation Test(go:GenericObj, so1:SpecificObj1, so2:SpecificObj2,
        so1a:SpecificObj1a) -> (boolean) =
(
    (*
     * The next three expressions show that the variable go of type GenericObj
     * is not directly compatible with variables of its subclass types.  In
     * order to obtain a value of a subclass type from go, the ?< and .<
     * operators can be used, as shown below.
     *)
    (go = so1)      (* ERROR: GenericObj is not type equiv to SpecificObj1 *)
        or
    (go = so2)      (* ERROR: GenericObj is not type equiv to SpecificObj2 *)
        or
    (so1 = so1a)    (* ERROR: SpecificObj is not type equiv to SpecificObj1a *)
        or

    (*
     * The next four expressions show how to use the ?< and .< operators to
     * test and select subclasss values from variable go.
     *)
    (go?<SpecificObj1 = true)   (* OK: go?<SpecificObj1 returns true if the
                                       current value of go is of type
                                       SpecificObj1. *)
        or
    (go.<SpecificObj1 = so1)    (* OK: go<.SpecificObj1 selects the value of
                                       type SpecificObj1 from go. *)
        or
    (so1.<SpecificObj1a = so1a) (* OK: so1.<SpecificObj1a extracts the value of
                                       SpecificObj1a from so1. *)
        or
    (go.<SpecificObj1.<SpecificObj1a = so1a);   (* OK *)
);