def alert!
      t = time.round( 9 )
      t2 = next_time.round( 9 )
      if t >= t2 then # it's time to sample
        sample!
        @next_time += sampling
      end
    end
def alert!
      t = time.round( 9 )
      t2 = next_time.round( 9 )
      if t >= t2 then # it's time to sample
        sample!
        @next_time += sampling
      end
    end
To be called by simulators whenever the state changes (every time that
 simulation +time+ is incremented).