[xsl] Param redirection

This is a question about how to program with XSLT.

I would like to have the following parameters in my stylesheet:

<xsl:param name="a_or_b"/>
<xsl:param name="a"/>
<xsl:param name="b"/>

If the value of param "a_or_b" is "a", I would like to output the value
of param "a".  If the value of param "a_or_b" is "b", I would like to
output the value of param "b".

How can I use the value of one param (i.e. "a_or_b") as the name of the
param for the value-of element?  Or is there another way to accomplish
this redirection in XSLT?

Thanks in advance for any assistance.

Regards,
Dave