':return: The previous cept of ``j``, or None if ``j`` belongs to
the first cept'

def previous_cept(self, j):

    i = self.alignment[j]
    if (i == 0):
        raise ValueError('Words    aligned    to    NULL    cannot    have    a    previous    cept    because    NULL    has    no    position')
    previous_cept = (i - 1)
    while ((previous_cept > 0) and (self.fertility_of_i(previous_cept) == 0)):
        previous_cept -= 1
    if (previous_cept <= 0):
        previous_cept = None
    return previous_cept
