code: def add(self, rule):
          
  
          assert isinstance(rule, ClassifierRule)
  
          condition = rule.condition
          action = rule.action
  
          # If the rule already exists in the population, then we virtually
          # add the rule by incrementing the existing rule's numerosity. This
          # prevents redundancy in the rule set. Otherwise we capture the
          # new rule.
          if condition not in self._population:
              self._population[condition] = {}
  
          if action in self._population[condition]:
              existing_rule = self._population[condition][action]
              existing_rule.numerosity += rule.numerosity
          else:
              self._population[condition][action] = rule
  
          # Any time we add a rule, we need to call this to keep the
          # population size under control.
          return self._algorithm.prune(self)
docs: Add a new classifier rule to the classifier set. Return a list
          containing zero or more rules that were deleted from the classifier
          by the algorithm in order to make room for the new rule. The rule
          argument should be a ClassifierRule instance. The behavior of this
          method depends on whether the rule already exists in the
          classifier set. When a rule is already present, the rule's
          numerosity is added to that of the version of the rule already
          present in the population. Otherwise, the new rule is captured.
          Note that this means that for rules already present in the
          classifier set, the metadata of the existing rule is not
          overwritten by that of the one passed in as an argument.
  
          Usage:
              displaced_rules = model.add(rule)
  
          Arguments:
              rule: A ClassifierRule instance which is to be added to this
                  classifier set.
          Return:
              A possibly empty list of ClassifierRule instances which were
              removed altogether from the classifier set (as opposed to
              simply having their numerosities decremented) in order to make
              room for the newly added rule.