Before revision:  Herein, the structure of the chain generating the stimuli is characterized by a rooted and labeled tree whose branches , henceforth called contexts, represent the sequences of past stimuli governing the choice of the next stimulus.
After revision:  Herein, the structure of the chain generating the stimuli are characterized by rooted and labeled tree whose branches , henceforth called contexts, represent the sequences of past stimuli governing the choice of the next stimulus.