code: public static Constraint differentialActivity(boolean activating)
  	{
  		if (activating) return new AND(new MappedConst(new ActivityConstraint(true), 1), new MappedConst(new ActivityConstraint(false), 0));
  		else return new AND(new MappedConst(new ActivityConstraint(true), 0), new MappedConst(new ActivityConstraint(false), 1));
  	}
docs: Gets a constraint to ensure that ensures only one of the two PhysicalEntities has an
  activity. Size of this constraint is 2.
  
  @param activating true/false (TODO explain)
  @return constraint to make sure only one of the PhysicalEntity has an activity