Start this Tracer.

        Return a Python function suitable for use with sys.settrace().

def start(self, origin):
        """
        Start this Tracer.

        Return a Python function suitable for use with sys.settrace().
        """
        self.start_time = time.time()
        self.pause_until = None
        self.data.update(self._get_struct(origin, 'origin'))
        self.data_stack.append(self.data)
        sys.settrace(self._trace)
        return self._trace