def form_neg_pair(info, mode='trn'):
    """
    Returns a list of 2 indices correspondign to index values
    of info which store different characters.
    """
    y = info[info.Mode == mode]
    curr_ind = y.Char.sample(1).index.values[0]
    curr_char = y.Char.loc[curr_ind]
    new_ind = y[~y.Char.isin([curr_char])].sample(1).index.values[0]
    return [curr_ind, new_ind]