Append data to coll
A reasonable way to append an item in the coll object is to keep track of the last or highest index. This can be done using a counter object or an i (int) object just above coll. Then increment the counter or i by one each time you add a new item to the coll object.
To store any message in a coll, you simply prepend an index number before the message. In this example, we are just storing integers from a number box. Whatever integer comes out of the number box gets held in the right inlet of a pack object, then we increment the index counter (the i object) by adding 1 to it and putting it back in the right inlet of the i object (thus always storing the highest index used), and we use that as the index to store the integer in the coll.