code: def backtrack(self, source):
          
          key = self.get_tok(source)
          s = self[key]()
          meta = s.metadata['original_source']
          cls = meta['cls']
          args = meta['args']
          kwargs = meta['kwargs']
          cls = import_name(cls)
          sout = cls(*args, **kwargs)
          sout.metadata = s.metadata['original_metadata']
          sout.name = s.metadata['original_name']
          return sout
docs: Given a unique key in the store, recreate original source