code: def validate(self):
          
          super().validate()
          nb_entities = len(self.entities)
          if nb_entities != self.rows + self.columns:
              raise self.error(
                  'Number of entities: %s != number of rows + '
                  'number of columns: %s+%s=%s' % (
                      nb_entities, self.rows, self.columns,
                      self.rows + self.columns))
docs: Base validation + entities = rows + columns.