anchor.diff:    }
   
     void update(double deltaT) {
  -    particle.applyImpluse(spring.force * deltaT);
  +    particle.applyImpulse(spring.force * deltaT);
       particle.update(deltaT);
       _applyInvariants();
     }
  
positive.code: 
    Particle({this.mass: 1.0, this.velocity: 0.0, this.position: 0.0});
  
    void applyImpluse(double impluse) {
      velocity += impluse / mass;
    }
  
    void update(double deltaT) {
  
negative.code: 
    void update(double deltaT) {
      double force = -_sign * friction;
      particle.applyImpulse(force * deltaT);
      if (particle.velocity.sign != _sign)
        particle.velocity = 0.0;
      super.update(deltaT);
  