def untrain(tokens, category)
      tokens = tokens.uniq if binarized
      data.decrement_examples(category)
      
      tokens.each do |token|
        if data.token_trained?(token, category)
          vocab.delete(token)
          data.remove_token_from_category(category, token)
        end
      end
    end
def untrain(tokens, category)
      tokens = tokens.uniq if binarized
      data.decrement_examples(category)
      
      tokens.each do |token|
        if data.token_trained?(token, category)
          vocab.delete(token)
          data.remove_token_from_category(category, token)
        end
      end
    end
Be carefull with this function:
 * It decrement the number of examples for the category.
   If the being-untrained category has no more examples, it is removed from the category list.
 * It untrain already trained tokens, non existing tokens are not considered.