code: def mapCellsToColumns(self, cells):
      
      cellsForColumns = defaultdict(set)
  
      for cell in cells:
        column = self.columnForCell(cell)
        cellsForColumns[column].add(cell)
  
      return cellsForColumns
docs: Maps cells to the columns they belong to.
  
      :param cells: (set) Cells
  
      :returns: (dict) Mapping from columns to their cells in `cells`