docs: def validate_square(data, message=None, exception=MatrixError):
      """Validate that the matrix has equal number of rows and columns."""
      rows, columns = len(data), len(data[0]) if data else 0
      if message is None:
          message = 'Number of rows: %s != number of columns: %s in matrix' % (
              rows, columns)
      if rows != columns:
          raise exception(message)