The join object lets you pack together various messages and lists into larger lists. Similar to the pack object, but input types are not predefined, it accepts lists as well as individual messages, and allows you to specify any inlet as a "hot" inlet to trigger the list output.
One great thing about join is that it mixes some of the functionality of pack and pak, allowing any inlet you specify to trigger output using the @triggers attribute (starting with 0 for the leftmost outlet); if you want all inlets to trigger (like pak) you can use @triggers -1.
If no message has been received at an inlet, it will default to outputting a 0 (int) for that position in the output list.
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