Observers

• FIXME

• push_simple.py: observables push notifications, execution is immediate
• push_buggy.py: infinite recursion
• push_origin.py: keep track of originator to avoid cycles
  • use UUID as identifier in exercise
• push_queue.py: use the graph to add work to a queue for execution
• delay_queue.py: split action from notification
• delay_queue_verbose.py: run with tracing
• logging.py: queue records actions and causes (better than verbose flag)