code: def add(self, name, nestable, **kw):
          
          self.checkpoints[name] = self.nest
          self.nest = copy.copy(self.nest)
          return self.nest.add(name, nestable, **kw)
docs: Adds a level to the nesting and creates a checkpoint that can be
          reverted to later for aggregation by calling :meth:`SConsWrap.pop`.
  
          :param name: Identifier for the nest level
          :param nestable: A nestable object - see
              :meth:`Nest.add() <nestly.core.Nest.add>`.
          :param kw: Additional parameters to pass to
              :meth:`Nest.add() <nestly.core.Nest.add>`.