static new HashMap<S, Class> findClass_cache;

// currently finds only inner classes of class "main"
// returns null on not found
// this is the simple version that is not case-tolerant
static Class findClass(String name) {
  synchronized(findClass_cache) {
    if (findClass_cache.containsKey(name))
      ret findClass_cache.get(name);
      
    if (!isJavaIdentifier(name)) ret null;
    
    Class c;
    try {
      c = Class.forName("main$" + name);
    } catch (ClassNotFoundException e) {
      c = null;
    }
    findClass_cache.put(name, c);
    ret c;
  }
}

The cache can contain null values - it's a very important optimization if you look for non-existing classes a lot (e.g. when loading foreign concepts without having the custom classes).