docs: def __find_index(alig_file_pth, idx_extensions):
    """
    Find an index file for a genome alignment file in the same directory.
  
    :param alig_file_path: path to the alignment file.
    :param idx_extensions: check for index files with these extensions
    :return: path to first index file that matches the name of the alignment file
             and has one of the specified extensions.
    """
    if idx_extensions is None:
      return None
    base, _ = os.path.splitext(alig_file_pth)
    for idx_ext in idx_extensions:
      candidate = base + os.extsep + idx_ext
      if os.path.isfile(candidate):
        return candidate
    return None