def is_at_del(row):
    """Encodes if the indel is an deletion with 'A' or 'T'
    
    Args:
        row (pandas.Series): a Series with 'is_ins'(bool)
                             and 'indel_seq'(str) indexes

    Returns:
        is_at_del (bool): 1 for 'A' or 'T' deletion
                          0 otherwise
    """
    if row["is_ins"] == 0 and row["indel_seq"] == "A":
        is_at_del = 1
    elif row["is_ins"] == 0 and row["indel_seq"] == "T":
        is_at_del = 1
    else:
        is_at_del = 0

    return is_at_del