Hello Joel, > 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? We have recently been more careful about naming variants, so that `\foo_bar:x` is always equivalent to `\exp_args:Nx \foo_bar:n`. Some `x` variants are optimized, for instance `\use:x`, but this function is still equivalent to `\exp_args:Nx \use:n` in all cases. There are a few situations where primitives provide x-like expansion: at the moment I can think of - `\write` and `\immediate \write`, which expand the text to be written when they write it. - `\pdfstrcmp`, which expands two brace groups and compares the resulting strings. - `\directlua`, which expands one brace group before turing it to a string and passing it to Lua. The full expansion that is performed by those primitives is different from normal `x`-expansion in a subtle way. `\use:x { ...##... }` gives only one macro parameter token `#`, and `\use:x { ...#... }` is an error, while with engine `x`-like expansion, `\pdfstrcmp{#}{}` is valid. There is also a much more obvious difference: `x`-expansion cannot be done expandably, meaning that it may not appear within another `x`-expansion, while several of the primitives are expandable. We decided to denote this engine-level `x`-expansion by ending the name part with `_x`, and to use `:n` as a signature. There is a precedent for such an `:n` signature for functions which perform some expansion: `\int_eval:n` and most `int` functions, as well as `dim`, `skip`, `fp`, etc. The `\lua_now:x` function used to be a wrapper for `\directlua`, which means that it was expandable and had a slightly different behaviour than `\exp_args:Nx \lua_now:n` when `#` were involved. This convinced us that it was misnamed, so we changed the name to `\lua_now_x:n` after checking on TeXLive that the function was unused (both Joseph and I missed the lualatex-math code). We also provided `\lua_now:x` as a true `\exp_args:Nx` variant of `\lua_now:n`, hence it became protected (non-expandable). Since lualatex-math.sty relied on the expandability (it had to), it unfortunately broke. Kind regards, Bruno