@staticmethod
def icod(nucl_seq, pos):
    """Retrieves the codon at the specified codon position from a nucleotide sequence

        Parameters
        ----------
        nucl_seq : str
        pos : int

        Returns
        -------
        str
            codon string of length 3

        """
    assert len(nucl_seq) % 3 == 0, ValueError(
        'Sequence is not in multiples of three.')
    assert isinstance(pos, int), ValueError(
        'Position should be an integer value.')
    return nucl_seq[3 * pos:3 * pos + 3]
