Vehicle variability example has incorrect constraint expression

[l-johnston]

The vehicle variability example has incorrect constraint expression in VehicleVariabilityModel::'150% Model'::PartsTree::vehicleFamily where engine is being tested against a part usage rather than a variant reference.

package MVCE {
    private import ScalarValues::*;

    part def Vehicle;
    part def Engine;

    part vehicle : Vehicle {
        part engine : Engine[1];
    }

    part engine : Engine;
    part '4cylEngine' :> engine;
    part '6cylEngine' :> engine;

    variation part def EngineChoices :> Engine {
        variant '4cylEngine';
        variant '6cylEngine';
    }

    abstract part lowcostVehicleFamily :> vehicle {
        part :>> engine : EngineChoices[1];

        assert constraint engineConstraint { engine == '4cylEngine' }
    }

    part vehicle4Cyl :> lowcostVehicleFamily {
        part :>> engine = engine::'4cylEngine';
    }
}

%eval MVCE::vehicle4Cyl.engineConstraint()

LiteralBoolean false (3aa994ba-2a68-4555-8e08-7b48d0133290)

package MVCE_Corrected {
    private import ScalarValues::*;

    part def Vehicle;
    part def Engine;

    part vehicle : Vehicle {
        part engine : Engine[1];
    }

    part engine : Engine;
    part '4cylEngine' :> engine;
    part '6cylEngine' :> engine;

    variation part def EngineChoices :> Engine {
        variant '4cylEngine';
        variant '6cylEngine';
    }

    abstract part lowcostVehicleFamily :> vehicle {
        part :>> engine : EngineChoices[1];

        assert constraint engineConstraint { engine == engine::'4cylEngine' }
    }

    part vehicle4Cyl :> lowcostVehicleFamily {
        part :>> engine = engine::'4cylEngine';
    }
}

%eval MVCE_Corrected::vehicle4Cyl.engineConstraint()

LiteralBoolean true (03008dbc-16f2-4a6d-90a7-7a2213f22044)