bike <| two_wheels.
bike <| vehicle.
truck <| four_wheels.
truck <| vehicle.
car <| four_wheels.
car <| vehicle.
glb(two_wheels,vehicle)=bike?
glb(four_wheels,vehicle)={car;truck}?
glb(two_wheels,four_wheels)={}?

glb(bike,vehicle)=bike?
glb(bike,@)=bike?
.
