def GetFragId(atdict, iat):
    """
    Obtain the fragment identifications from the atom description

    Args:
      atdict(dict): dictionary of the atom
      iat (int): position of the atom in the list

    Returns:
      str: fragment_id
    """
    fragid = atdict.get('frag', 'ATOM:' + str(iat))

    if all(':' in elem for elem in fragid):
        fragid = '+'.join(map(str, fragid))
    elif len(fragid) == 2 and ':' in str(fragid[1]):
        fragid = '-'.join(map(str, fragid))
    elif isinstance(fragid, list):
        fragid = ':'.join(map(str, fragid))
    return fragid