In the Actions list in the Index, I see:
“trace”, a testing command not available in the final game
I couldn’t figure out what it does. 
As Graham puts it:
Not all of these commands are documented; this is intentional. They may be changed in name or function.
But, even if they’re undocumented, they exist, and can be extremely useful!
TRACE sets the level of “parser tracing”, which shows the parser’s internal workings as it processes each command. TRACE 0 means no tracing (the default) while TRACE 5 means way more detail than you’d ever want. Usually you’ll want to set it to something in the middle.
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BTW it goes to ‘TRACE 6’ which has even more detail than that. (TRACE 6 gives you all the DSA lines which are “do scope action” or something like that.)