def sort_by(all_tss_pos, all_tts_pos):
    """
    Sort either by TSS positions or TTS ones
    depending on where is the minimum value
    Args:
        all_tss_pos (list): list of all TSS position extracted from the 'tss.dat' file.
        all_tts_pos (list): list of all TTS position extracted from the 'tts.dat' file.
    Returns:
        col_name (string) : which will be either "TSS_pos" or "TSS_pos"
    """
    min_tss = min(all_tss_pos)
    min_tts = min(all_tts_pos)
    col_name = ""
    if min_tss < min_tts:
        col_name = "TSS_pos"
    else:
        col_name = "TTS_pos"
    return col_name