Traceback (most recent call last):
  File "img_model.py", line 263, in <module>
    tr.optimize()
  File "/home/kilian/tractor/tractor/engine.py", line 784, in optimize
    allderivs = self.getDerivs()
  File "/home/kilian/tractor/tractor/engine.py", line 917, in getDerivs
    assert(False)
AssertionError