actions:
    crossTogether(X,Y) requires person(X), person(Y), X < Y
                       costs Smax where speed(X,SX), speed(Y,SY), 
                                        max(SX,SY,Smax). 
    cross(X) requires person(X) costs SX where speed(X,SX).
    giveLampTo(X) requires person(X).

fluents: 
    across(X) requires person(X).
    hasLamp(X) requires person(X).
    differentSides(X,Y) requires person(X), person(Y). 

initially: 
    -across(X).
    hasLamp(a).

always: 
    executable crossTogether(X,Y) if hasLamp(X).
    executable crossTogether(X,Y) if hasLamp(Y).
    nonexecutable crossTogether(X,Y) if differentSides(X,Y).

    executable cross(X) if hasLamp(X).

    executable giveLampTo(X).
    nonexecutable giveLampTo(X) if hasLamp(Y), differentSides(X,Y).

    caused across(X) after crossTogether(X,Y), -across(X).
    caused across(Y) after crossTogether(X,Y), -across(Y).
    caused -across(X) after crossTogether(X,Y), across(X).
    caused -across(Y) after crossTogether(X,Y), across(Y).

    caused across(X) after cross(X), -across(X).
    caused -across(X) after cross(X), across(X).

    caused hasLamp(X) after giveLampTo(X).
    caused -hasLamp(X) after giveLampTo(Y), X !=Y, hasLamp(X).

    caused differentSides(X,Y) if across(X), -across(Y).
    caused differentSides(X,Y) if -across(X), across(Y).

    inertial across(X).
    inertial -across(X).
    inertial hasLamp(X).

noConcurrency.

goal: across(a), across(b), across(c), across(d)?