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 row_col(self, ind):
        i = 0
        for r in range(self.nr):
            for c in range(self.nc):
                if i == ind:
                    return r, c
                i += 1