code: public static PhysicsEngine setPhysicsEngine(PhysicsEngine newEngine) {
  		final PhysicsEngine oldEngine = engine;
  		if (newEngine == null) {
  			engine = new JavaPhysicsEngine();
  		} else {
  			engine = newEngine;
  		}
  		return oldEngine;
  	}
docs: Set the current physics engine.
  
  <p>If the given engine is <code>null</code>, the default physics engine
  is used (java implementation).
  
  @param newEngine the current physics engine, or <code>null</code> for default engine.
  @return previous physics engine.