Some additions to an _instance package for instances of the kind "backdrop".

§1. Compilation data. This additional data is present only if the "backdrops" feature is active: 

inter_name *RTBackdropInstances::found_in_val(instance *I, int many) {
    if (BACKDROPS_DATA(I)->found_in_fn_iname == NULL)
        BACKDROPS_DATA(I)->found_in_fn_iname =
            Hierarchy::make_iname_in(BACKDROP_FOUND_IN_FN_HL, RTInstances::package(I));
    BACKDROPS_DATA(I)->many_places = many;
    return BACKDROPS_DATA(I)->found_in_fn_iname;
}

§2. Compilation. We add a found_in function to test whether the given backdrop is found in the current location or not. 

void RTBackdropInstances::compile_extra(instance *I) {
    if ((K_backdrop) && (Instances::of_kind(I, K_backdrop)) &&
        (BACKDROPS_DATA(I)->found_in_fn_iname)) {
        if (BACKDROPS_DATA(I)->many_places)
            The object is found in many rooms or in whole regions2.1
        else
            The object is found nowhere2.2;
    }
}

§2.1. The object is found in many rooms or in whole regions2.1 = 

    packaging_state save = Functions::begin(BACKDROPS_DATA(I)->found_in_fn_iname);
    inference *inf;
    POSITIVE_KNOWLEDGE_LOOP(inf, Instances::as_subject(I), found_in_inf) {
        instance *loc = Backdrops::get_inferred_location(inf);
        EmitCode::inv(IF_BIP);
        EmitCode::down();
        if ((K_region) && (Instances::of_kind(loc, K_region))) {
            EmitCode::call(Hierarchy::find(TESTREGIONALCONTAINMENT_HL));
            EmitCode::down();
                EmitCode::val_iname(K_object, Hierarchy::find(LOCATION_HL));
                EmitCode::val_iname(K_object, RTInstances::value_iname(loc));
            EmitCode::up();
        } else {
            EmitCode::inv(EQ_BIP);
            EmitCode::down();
                EmitCode::val_iname(K_object, Hierarchy::find(LOCATION_HL));
                EmitCode::val_iname(K_object, RTInstances::value_iname(loc));
            EmitCode::up();
        }
            EmitCode::code();
            EmitCode::down();
                EmitCode::rtrue();
            EmitCode::up();
        EmitCode::up();
    }
    EmitCode::rfalse();
    Functions::end(save);

§2.2. The object is found nowhere2.2 = 

    packaging_state save = Functions::begin(BACKDROPS_DATA(I)->found_in_fn_iname);
    EmitCode::rfalse();
    Functions::end(save);