code: def tc_at_grid_col(self, idx):
          
          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')
docs: The ``<w:tc>`` element appearing at grid column *idx*. Raises
          |ValueError| if no ``w:tc`` element begins at that grid column.