code: def add(self, cell, overwrite_duplicate=False):
          
          if isinstance(cell, Cell):
              if (not overwrite_duplicate and cell.name in self.cell_dict and
                      self.cell_dict[cell.name] is not cell):
                  raise ValueError("[GDSPY] cell named {0} already present in "
                                   "library.".format(cell.name))
              self.cell_dict[cell.name] = cell
          else:
              for c in cell:
                  if (not overwrite_duplicate and c.name in self.cell_dict and
                          self.cell_dict[c.name] is not c):
                      raise ValueError("[GDSPY] cell named {0} already present "
                                       "in library.".format(c.name))
                  self.cell_dict[c.name] = c
          return self
docs: Add one or more cells to the library.
  
          Parameters
          ----------
          cell : ``Cell`` of list of ``Cell``
              Cells to be included in the library.
          overwrite_duplicate : bool
              If True an existing cell with the same name in the library
              will be overwritten.
  
          Returns
          -------
          out : ``GdsLibrary``
              This object.