1. A method for testing a device which generates output in response to external stimuli, said method comprising the steps of: (a) generating a first set of observed output from the device by applying a first set of external stimuli to the device; (b) generating a first set of model output by applying the first set of observed output to a model of the device; (c) setting at least one verification condition which the model output should satisfy; and (d) determining if the model output does not satisfy one of the verification conditions.