before a pico is deleted, its subscriptions should be deleted

[b1conrad]

otherwise it becomes impossible to remove the subscription from the other pico, even though it still remains

[b1conrad]

Temporarily, I treated the wrangler:subscription_cancellation event as if it involved a different pico-engine. And I was able to remove such a subscription.

See issue #498 which may be related

[b1conrad]

Proposed this way to fix it, to a CS462 student:

ruleset subscription_fixes {
  rule emergencyFix {
    select when emergency dangling_subscription
      Id re#(.+)#
    fired {
      raise wrangler event "established_removal" attributes event:attrs
    }
  }
}

to be installed in the pico with the dangling subscription, and invoked from the Testing tab, passing in the Id of the incomplete subscription (available in either the Logging tab of a failed deletion attempt, or simply in the Subscriptions tab).

[b1conrad]

Looks like the attribute "Id" isn't used in the rule. BUT. It needs to be mentioned so that the compiler will generate the right thing for the Testing tab.