docs: def is_sparse_file(filename):
      """Determine if the given filename indicates a dense or a sparse matrix
  
         If pathname is xxx.coo.yyy return True otherwise False.
  
      """
      dirname, basename = os.path.split(filename)
      name, ext = os.path.splitext(basename)
      matrix_name, matrix_ext = os.path.splitext(name)
      if matrix_ext == '.coo':
          return True
      else:
          return False