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Having a pico that maintains a collection of other picos is a pattern that is often useful.

...

  rule collection_subscriptions {
    select when wrangler child_initialized
    pre {
      eci = event:attr("eci");
      collection_eci = get_wellKnown_eci(eci);
    }
    if collection_eci then noop();
    fired {
      raise grouper event "need_subscriptions"
        attributes event:attrattrs("rs_attrs").put("collection_eci",collection_eci);
    }
  }

...

This KRL code will respond to the internal event, expecting the collection_eci and the child_specs given to wrangler earlier (wrangler renames it rs_attrs for some reason).

  rule collection_subscription_requests {
    select when grouper need_subscriptions
    foreach by_prefix(event:attr("name_prefix")) setting(room_name,tag_id)
    pre {
      eci = building:eci(tag_id);
      wellKnown_Tx = get_wellKnown_eci(eci).klog("wellKnown_Tx");
    }
    if wellKnown_Tx then every {
      event:send({"eci":eci,"domain":"wrangler","type":"subscription",
        "attrs": { "wellKnown_Tx": event:attr("collection_eci"),
          "Rx_role": "member", "Tx_role": "collection",
          "name": tag_id, "channel_type": "subscription" }
      })
    }
  }

...

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
    }
  }