code: def do_node_set(self, element, decl, pseudo):
          
          target = serialize(decl.value).strip()
          step = self.state[self.state['current_step']]
          elem = self.current_target().tree
          _, valstep = self.lookup('pending', target)
          if not valstep:
              step['pending'][target] = [('nodeset', elem)]
          else:
              self.state[valstep]['pending'][target] = [('nodeset', elem)]
docs: Implement node-set declaration.