def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def dec_c(self, ind):
        r, c = self.row_col(ind)
        c -= 1
        if c < 0:
            c = self.nc - 1
        if r == (self.nr - 1) and c == (self.nc - 1):
            c = self.nc - 2
        return self.indx(r, c)