can't be that difficult to implement a suitable \superscript macro. Indeed.
I committed the changes to texinfo.tex (and the documentation) so that @sub/@sup stay in math mode inside @math, and do a textual sub/superscript outside @math. The remaining complication (unsolved) is if someone wants a word as a sub/superscript inside @math, like my original example, @math{x@sub{first}} You suggested using @asis, but that doesn't make sense to me; @asis should just mean, well, as-is. @r works, more or less by accident, but @i (what one would typically want) doesn't, for two reasons: 1) @math switches to (99% of) @tex mode, where @i = plain \i = dotless i. That would be easy enough (though undesirable in principle, it seems to me) to kludge around, but then there is (2) the TeX font families aren't all set up since we haven't needed them before. It all gets pretty painful, and it seems an unlikely usage, so I'm hoping we can just leave it unsolved for now. k