[XeTeX] Single glyphs from a font

[Joel C. Salomon]

Joel C. Salomon wrote:
> I’m trying to create this effect:
>     a² + b² = c²   (1)
>     x² + y² = z²   (1′)
>   

I think I figured it out.  In my preamble I have:
    \newfontface\cambriamath{Cambria Math}
    \newcommand{\textprime}{\cambriamath ′}
Then the equation looks like:
    \begin{equation} \label{abc}
       a^2 + b^2 = c^2
    \end{equation}
    \begin{equation*} \label{abc:prime} \tag{\ref*{abc}\textprime}
       x^2 + y^2 = z^2
    \end{equation*}
This works, but need I be wary of font-change commands in \tag{}s?

Also, why does
    \tag{\ref*{abc}$′$}
not work? The glyph seems to be treated as a no-op in this context.

—Joel Salomon