string-calculated entity reference

Is is possible to string-calculated entity reference?
what i mean is instead of saying &a; or &b;,
i want something like &x; where x is calculated to be a or b.
is this possible?

i have now this:

<!DOCTYPE stylesheet [
<!ENTITY a "hello">
<!ENTITY b "bye">
]
<xsl:stylesheet>
  
<xsl:template match="a"/>
 &a;
</xsl:template>

<xsl:template match="b"/>
 &b;
</xsl:template>

</xsl:stylesheet>


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