Imagine you are designing a virtual world in which the following types of objects needed to be inter-related: lamps, switches, outlets, and rooms. Assume the following semantics for those objects:
a. A switch is either on or off and there is one switch per room.
b. When a switch is off, none of the outlets in the same room have power, but when the switch is on all of them do.
c. A lamp may be plugged into exactly one outlet, and an outlet may have 0, 1, or 2 lamps plugged into it.
d. A lamp is lit when it is plugged into an outlet that has power, and otherwise is unlit.