docs: def cells(self):
          """The number of cells in the MOC.
  
          This gives the total number of cells at all orders,
          with cells from every order counted equally.
  
          >>> m = MOC(0, (1, 2))
          >>> m.cells
          2
          """
  
          n = 0
  
          for (order, cells) in self:
              n += len(cells)
  
          return n