anchor.diff:    
     Physics() {
       space = new BasicSpace(this);
  -    emitter = new Emitter(this); 
  +    emitter = new Emitter<P>(this); 
     }
     
     addEffector(Effector effector, [int state=Particle.ALIVE]) {
  
positive.code: class Physics<P extends Particle, S extends Spring>
  {  
    Space space;
    Emitter emitter;
    List<P> particles = [];
    List<S> springs = [];
    
  
negative.code: /**
   * Base class for all physical behaviour, forces and constraint effectors.
   */
  abstract class Effector<P extends Particle> 
  {
    prepare() {}
    apply(P p);
  }
  
  abstract class Behaviour extends Effector {}
  