Subscript special chars needs braces

[trevorcampbell]

I'm not sure if this is so important -- not even sure if I want to change this -- but figured I'd document it here. Currently,

f_\scS

will cause LaTeX to hit an error:

[21]
! Missing { inserted.
<to be read again> 
                   \let 
l.726 \]

In contrast:

f_{\scS}

works.