def _codon_slicing(nt_pos):
    """Return (AA_position, codon_index_start, codon_index_stop)"""

    aa_pos, reminder = divmod(nt_pos, 3)
    if reminder == 0:
        start = nt_pos - 3
    else:
        start = nt_pos - reminder
        aa_pos+=1
    return (aa_pos, start, start + 3)