#### Extending `OZ_Expect`

Propagators are imposed on their parameters by foreign functions which are invoked by the Oz runtime system. Such foreign functions use the CPI class `OZ_Expect` to check that the parameters are appropriately kinded (resp. constrained) or represent compatible values. The class `OZ_Expect` provides the member function

> `OZ_expect_t OZ_Expect::expectGenCtVar(OZ_Term t,
> OZ_CtDefinition * d,
> OZ_CtWakeUp w);`

to define appropriate expect-functions, e.g., for real-interval constraints. The customized class defines member functions that check for real-intervals and determine the wake-up event. To do that, the static members functions of `RIWakeUp` (see Section ``Determining Wake-up Events for Real-Interval Constraints'') are used and the global variable `RIDefinition ri_definition` is assumed.

>     class RIExpect : public OZ_Expect {
>     public:
>       OZ_expect_t expectRIVarMin(OZ_Term t) {
>         return expectGenCtVar(t, ri_definition,
>                               RIWakeUp::wakeupMin());
>       }
>       OZ_expect_t expectRIVarMax(OZ_Term t) {
>         return expectGenCtVar(t, ri_definition,
>                               RIWakeUp::wakeupMax());
>       }
>       OZ_expect_t expectRIVarMinMax(OZ_Term t) {
>         return expectGenCtVar(t, ri_definition,
>                               RIWakeUp::wakeupMinMax());
>       }
>       ...
>     };

The class `RIEexpect` can now be used to define foreign functions that impose propagators on their parameters.

> `OZ_BI_define(ri_lessEq, 2, 0)
> {
> OZ_EXPECTED_TYPE("real interval, real interval");
> RIExpect pe;
> OZ_EXPECT(pe, 0, expectRIVarMinMax);
> OZ_EXPECT(pe, 1, expectRIVarMinMax);
> return pe.impose(new RILessEq(OZ_args[0],
> OZ_args[1]));
> }
> OZ_BI_end`

The propagator class `RILessEq` is partly defined next.