Should we have ${Xmeta as ident} ?

This is straightforward to implment. But it may be confusing in use.

In most places as tokens will do (with perhaps less good error handling).

Within ${paste } you usually want to use as str because you usually (i) don't want to exclude keywords (you're going to paste, dekeywording them) (ii) you want to accept middle fragments (ie values which eg start with a digit).

Options:

1. Implement this and document (and test) the corner cases
2. Implement it but (a bit weirdly) forbid as ident in ${paste ...}
3. Implement it but make as ident in ${paste ...} accept keywords and middle fragments (maybe call it identfrag?)
4. Don't implement this and put a comment in enum meta::SubstAs explaining why not to