code: <T extends JCTree> List<Type> classEnter(List<T> trees, Env<AttrContext> env) {
          ListBuffer<Type> ts = new ListBuffer<>();
          for (List<T> l = trees; l.nonEmpty(); l = l.tail) {
              Type t = classEnter(l.head, env);
              if (t != null)
                  ts.append(t);
          }
          return ts.toList();
      }
docs: Visitor method: enter classes of a list of trees, returning a list of types.