[XeTeX] Question about \fontspec_set_family function of fontspec

[VAFA KHALIGHI]

If I define a font command as follow: (using the new fontspec version)

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\ExplSyntaxOn

 \DeclareDocumentCommand \setmyfont { O{} m } {
\fontspec_set_family:Nnn \myfont {Mapping=tex-text,#1}{#2}
}
\ExplSyntaxOff
\AtBeginDocument{\myfont}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


and choose a font say \setmyfont[ExternalLocation]{....}

then instead typesetting text in this font, only the name of the font will
be printed. Why?

Thanks
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