Output format specifier for post index

[jb2170]

I've spent like 20 minutes looking through documentation / googling and I still can't find the output format specifier for post index within a list!

Something like gallery-dl 'https://twitter.com/user' -f '{index}_{id}.{extension}', but not index

What should index be to get the enumeration index of the post within the list of things being downloaded

[the-blank-x]

~~It should be {num} (and there's also a corresponding {count} keyword)~~
Edit: I think I misinterpreted your request--I believe you want an index for each tweet from a user (e.g. the first tweet in a list is index 1, the second index 2, etc)?