code: public Queue<Env<AttrContext>> attribute(Queue<Env<AttrContext>> envs) {
          ListBuffer<Env<AttrContext>> results = new ListBuffer<>();
          while (!envs.isEmpty())
              results.append(attribute(envs.remove()));
          return stopIfError(CompileState.ATTR, results);
      }
docs: Attribute a list of parse trees, such as found on the "todo" list.
  Note that attributing classes may cause additional files to be
  parsed and entered via the SourceCompleter.
  Attribution of the entries in the list does not stop if any errors occur.
  @return a list of environments for attribute classes.