pub trait MessageSet: MessageVariantSet {
type Handler: 'static + HandleAsync<Self::AsyncVariant> + HandleSync<Self::SyncVariant> + HandleAsyncConcurrent<Self::AsyncConcurrentVariant> + HandleSyncConcurrent<Self::SyncConcurrentVariant>;
type Async;
type Sync;
type AsyncConcurrent;
type SyncConcurrent;
}