The ``<w:tc>`` element appearing at grid column *idx*. Raises
        |ValueError| if no ``w:tc`` element begins at that grid column.

def tc_at_grid_col(self, idx):
        """
        The ``<w:tc>`` element appearing at grid column *idx*. Raises
        |ValueError| if no ``w:tc`` element begins at that grid column.
        """
        grid_col = 0
        for tc in self.tc_lst:
            if grid_col == idx:
                return tc
            grid_col += tc.grid_span
            if grid_col > idx:
                raise ValueError('no cell on grid column %d' % idx)
        raise ValueError('index out of bounds')