How can you tell if a variable exists?

How can you tell if a variable exists (or rather does not exist), without
erroring out of the stylesheet.

For example:
  I am using the SAXON servlet to present HTML to the user.  I am passing in
the parameters from doPost to the stylesheet.  If the parameters exist, all
is well.  But when a parameter does not exist, and is not passed to the
stylesheet, I just get an exception.  How do I prevent that?

TIA,
Scott Sanders


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list