code: def get(self, rule, default=None):
          
          assert isinstance(rule, ClassifierRule)
  
          if (rule.condition not in self._population or
                  rule.action not in self._population[rule.condition]):
              return default
          return self._population[rule.condition][rule.action]
docs: Return the existing version of the given rule. If the rule is
          not present in the classifier set, return the default. If no
          default was given, use None. This is useful for eliminating
          duplicate copies of rules.
  
          Usage:
              unique_rule = model.get(possible_duplicate, possible_duplicate)
  
          Arguments:
              rule: The ClassifierRule instance which may be a duplicate of
                  another already contained in the classifier set.
              default: The value returned if the rule is not a duplicate of
                  another already contained in the classifier set.
          Return:
              If the rule is a duplicate of another already contained in the
              classifier set, the existing one is returned. Otherwise, the
              value of default is returned.