@classmethod
def build_ixarr(cls, iarr, jarr):
    """
        Build an array of indexes. Each value represent a pair of letters.
        :param iarr: a 1d array representing a string
        :param jarr: a 1d array representing a string
        :return: a 2d array
        """
    iarr = iarr.astype('uint16') << 8
    jarr = jarr.astype('uint16')
    iarr[0] = 0
    jarr[0] = 0
    iarr.shape = -1, 1
    jarr.shape = 1, -1
    return iarr | jarr
