':return: The position of the previous word that is in the same
tablet as ``j``, or None if ``j`` is the first word of the
tablet'

def previous_in_tablet(self, j):

    i = self.alignment[j]
    tablet_position = self.cepts[i].index(j)
    if (tablet_position == 0):
        return None
    return self.cepts[i][(tablet_position - 1)]
