final PropagationTrigger trigger; // an object that starts the propagation
public TwoBucketPropagationEngine(Solver solver) {
this.exception = new ContradictionException();
this.environment = solver.getEnvironment();
this.trigger = new PropagationTrigger(this, solver);
variables = solver.getVars();
List<Propagator> _propagators = new ArrayList<>();
Constraint[] constraints = solver.getCstrs();
int nbProp = 0;