Comment by carlehewitt

Every Actor has a region of mutual exclusion.

However, the region of mutual exclusion can have holes so that

    * activities can be suspended and later resumed
    * other activities can use the region of mutual
      exclusion while a message is being processed by
      another Actor

For example, a readers/writer scheduler for a database must be processing multiple activities concurrently, which is very difficult to implement in Erlang.