sort on param value

How can I perform an <xsl:sort> sort on an attribute whose name is given by a 
parameter value?

<xsl:param name="sort" />
...
<xsl:sort select="???" />

I've tried select="$sort" which does nothing and select="{$sort} and 
select="@{$sort} which both cause errors (along the lines of invalid syntax).

The parameter value passed is the name of the attribute I want to sort on with 
an at symbol already prefixed, e.g. "@name". (Though the at symbol could be 
left off it helps to solve the problem.)

Cheers,
Richard


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