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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def update(self, dt=None):
 """Simulate the model for a given time interval. Parameters dt : Optional[float] The time step to simulate, if None, the default built-in time step is used. """

        # EMELI passes dt = -1 so we need to handle that here
        dt = dt if (dt is not None and dt > 0) else self.dt
        tspan = [0, dt]
        # Run simulaton with initials set to current state
        res = self.sim.run(tspan=tspan, initials=self.state)
        # Set the state based on the result here
        self.state  = res.species[-1]
        self.time += dt
        if self.time > self.stop_time:
            self.DONE = True
        print((self.time, self.state))
        self.time_course.append((self.time.copy(), self.state.copy()))