[ODE] Angle rotation

[Alexander Bussman]

Hi!
(This might be off topic, but I hope it's ok)
I have recently started a simple project using ODE, but I have a problem:
I need to get the current angle of a object's rotation.

What I do now to rotate a object (that is rotating a object manually):
  dQuaternion q;
  dGeomGetQuaternion(obj->GetGeom(), q);
  dQFromAxisAndAngle(q, ax, ay, az, angle);
  obj->SetQuaternion(q);

(obj is a object of my ode object class)
angle is a variable that I call the rotate function with (also ax, ay and az).

So what I want is to do something like this:
  dQuaternion q;
  dGeomGetQuaternion(obj->GetGeom(), q);
  dReal newAngle = obj->GetAngle()+angle; // <---- something like that
  dQFromAxisAndAngle(q, ax, ay, az, newAngle);
  obj->SetQuaternion(q);

Anyone who could give me a hint on how I could solve my problem?

// Alexander Bussman