[luatex] Use Math Glyphs from Text Font

[C. Comren]

> \newfontface\myfont{MyFont}
> \makeatletter
> \DeclareMathSymbol{A}{\mathalpha}{\zf at family}{`A}
> \makeatother
>
> or (probably better)
>
> \ExplSyntaxOn
> \fontspec_set_family:Nnn \myfont {} {MyFont}
> \ExplSyntaxOff
> \DeclareMathSymbol{A}{\mathalpha}{\myfont}{`A}

Thanks for your suggestion. However, I get the error message

! LaTeX Error: Symbol font `MyFont(0)' is not defined.

Thanks,
CC