'Add a new variable-value pair to the assignment, and update
``self.variant``.'

def add(self, var, val):

    assert (val in self.domain), (u'%s    is    not    in    the    domain    %s' % (val, self.domain))
    assert is_indvar(var), (u"Wrong    format    for    an    Individual    Variable:    '%s'" % var)
    self[var] = val
    self._addvariant()
    return self
