code: def validate_categories_equal_entities(categories, entities, message=None,
                                         exception=MatrixError):
      
      nb_categories = len(categories)
      nb_entities = len(entities)
      if message is None:
          message = 'Number of categories: %s != number of entities: %s' % (
              nb_categories, nb_entities)
      if categories and nb_categories != nb_entities:
          raise exception(message)
docs: Validate that the matrix has equal number of entities and categories.