[NEW] Symbolic tags on single-line equations

[Rob Beezer]

New feature, possible only as a consequence of the big changes to display math
elements. The old #me and #men did not support the @tag attribute, like
an #mrow does. Now you can place a @tag attribute onto an #md that has no
#mrow children. And you will be able to specify, and use, various symbolic
tags like stars, or daggers, or crosses, as you have alwys been able to do for
multi-line display mathematics.

Rob