On 05/20/2014 09:50 AM, David Carlisle wrote:
> One thing I meant to say, is if  \mathsf{abc} is implemented by
> switching to a sans font.
> What to do about
>
> $ U+1D5BA  U+1D5BB U+1D5BC $
>
> that is
>
> MATHEMATICAL SANS-SERIF SMALL [ABC]

I've been eavesdropping, and what I'm saying is
essentially what others have said in various ways,
but seems worth reiterating.

To _me_ it seems rather fundamental that the various
\mathsf, etc, not only request a family/shape/slant/whatever,
but represent a single mathematical token.
The spacing around it would be appropriate for ORD (normally, I guess),
but within, the contents are essentially text (kerned, ligature, etc),
not treated as a sequence of math symbols, independently of whether the
eventual font turns out to be a "math" font.

So, while David's 2 examples might very well choose the same
font, I would expect the spacing around & between to be completely 
different.

bruce