I’m following the LaTeX3 bug tracker, and saw the issue about
`\lua_now:x` being renamed to `\lua_now_x:n`.

I suspect the explanation will be over my head, but since I haven't
seen this discussed on this list or explained in the l3 docs I figure
someone's got to ask:

What's the difference between the `:x` signature and the `_x:n`
suffix+signature?

—Joel