Inasmuch as it is probable that the pattern and detail of the machinery for mutual aid among two or more states may differ from that among the states that are party hereto, this compact contains elements of a broad base common to all states, and nothing herein shall preclude any state entering into supplementary agreements with another state or affect any other agreements already in force between states. Supplementary agreements may include, but shall not be limited to, provisions for evacuation and reception of injured and other persons and the exchange of medical, fire, police, public utility, reconnaissance, welfare, transportation and communications personnel, and equipment and supplies.