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and specify additional processing in some_function (defined in lines 6-8) that it shares (in line 3) to endpoints, and/or specify rules like some_rule (defined in lines 10-14) that take action on the collection's members when certain events occur.

When a member is added

Since members of the collection are determined by subscriptions, your ruleset can listen for wrangler:subscription_added events to do something when a new member joins your collection. Since your pico might participate in other subscriptions, only those in which the roles are "collection" and "member" would be applicable.

  rule new_member {
    select when wrangler subscription_added
    pre {
      pertinent = event:attr("Rx_role")=="collection"
                && event:attr("Tx_role")=="member";
    }
    if pertinent then noop();
    fired {
      raise collection event "new_member" attributes event:attrs
    }
  }

The event attributes available are those surrounding a subscription in general. Some of these might be useful when a new member arrives into your collection

When a member is removed

Your ruleset might wish to listen to the event wrangler:subscription_removed which will be raised when a subscription is removed from your pico. Such events which are applicable to membership in your collection could then be handled appropriately.

  rule new_member {
    select when wrangler subscription_removed
    pre {
      pertinent = event:attr("Rx_role")=="collection"
                && event:attr("Tx_role")=="member";
    }
    if pertinent then noop();
    fired {
      raise collection event "member_removed" attributes event:attrs
    }
  }