code: def require(self, *namespecs):
          
          blacklist = set([n[1:] for n in namespecs if n.startswith('!')])
          not_blacklist = [n for n in namespecs if not n.startswith('!')]
          return Bundle(*[bundle for name, version, bundle
              in self._require_recursive(*not_blacklist) if name not in blacklist])
docs: Return a bundle of the requested assets and their dependencies.