@Override
  public void applyTo(SubSystem s) {
    cdt = c / s.getDt();

    s.addToA(aPin, aPin, cdt);
    s.addToA(aPin, bPin, -cdt);
    s.addToA(bPin, bPin, cdt);
    s.addToA(bPin, aPin, -cdt);
  }
 @Override
 public void addedTo(SubSystem s) {
   super.addedTo(s);
   s.addProcess(this);
 }
 @Override
 public void simProcessI(SubSystem s) {
   double add = (s.getXSafe(aPin) - s.getXSafe(bPin)) * cdt;
   s.addToI(aPin, add);
   s.addToI(bPin, -add);
 }