def _lsb_2fold(aa, bit):
    """
    This function embeds a pair of bits in 2/3 fold degenerative codon.
    :param aa: amino acid information.
    :param bit: bit (character 2 e.g. 0) which should be embedded in codon.
    :return: watermarked codon (string) e.g. AGA.
    """
    if bit == '0':
        return aa["codons"][0]
    else:
        return aa["codons"][1]