def mapCellsToColumns(self, cells):
cellsForColumns = defaultdict(set)<EOL>for cell in cells:<EOL><INDENT>column = self.columnForCell(cell)<EOL>cellsForColumns[column].add(cell)<EOL><DEDENT>return cellsForColumns<EOL>
Maps cells to the columns they belong to.

:param cells: (set) Cells

:returns: (dict) Mapping from columns to their cells in `cells`