static PyObject *speed(PyObject *self, PyObject *args) {
  float value = -1.0f;
  if (!PyArg_ParseTuple(args, (char *)"|f",&value))
    return NULL;
 
  VMDApp *app = get_vmdapp();
  if (value > 0) { 
    app->animation_set_speed(value);
  } 
  
  // always return current value
  return PyFloat_FromDouble(app->anim->speed());
}
static void speed_cb(Fl_Widget *w, void *v) {
  Fl_Slider *slider = (Fl_Slider *)w;
  VMDApp *app = (VMDApp *)v;
  app->animation_set_speed((float) slider->value());
}