[Rd] Misuse of $<matn expressions>$ in Rd files

[Peter Dalgaard]

Prof Brian Ripley wrote:
> 'Writing R Extensions' tells you that $ needs to be escaped in Rd files 
> outside \code and similar.  So I was surprised to find that ca 80 CRAN 
> packages have constructions like (from ISwR)
> 
>   \item{\code{folate}}{
>     a numeric vector, folate concentration ($\mu$g/l).
>   }
> 
> This does not render sensibly in non-latex conversions, and it is what 
> we have \eqn{} for.

I suspect last I looked at this was in copy editing that getting the 
LaTeX to look right took priority.

It renders as "$\mu$g/l" (for now) which is semi-sensible to my eyes. I 
can see that I have \eqn{\mu}g/l in other places, but that comes out as 
"mug/l" which looks like an inverted measure for coffee cups. I probably 
gave up deciding a good incantation for "microgram per liter" in text 
mode. Possibly \eqn{\mu}{u} is the best on can do.

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