code: private void appendNonExistentCallbacks(HandlerAndTask from, HandlerAndTask to) {
      final Set<ProxyImplContent> proxyImplContentsTo = implMap.get(to);
      if (proxyImplContentsTo == null) {
        return;
      }
  
      final Set<ProxyImplContent> proxyImplContentsFrom = implMap.get(from);
      if (proxyImplContentsFrom == null) {
        return;
      }
  
      for (ProxyImplContent proxyImplContentFrom : proxyImplContentsFrom) {
        boolean exists = false;
        for (ProxyImplContent proxyImplContentTo : proxyImplContentsTo) {
          if (proxyImplContentTo.annotation.equals(proxyImplContentFrom.annotation)) {
            exists = true;
            break;
          }
        }
  
        if (!exists) {
          proxyImplContentsTo.add(proxyImplContentFrom);
        }
      }
    }
docs: Copy all callbacks implementations from -> to the specified set. It makes sure
  to copy the callbacks that don't exist already on the destination set.
  
  @param from key for the set of callbacks to copy from
  @param to   key for the set of callbacks to copy to