XSLT: check existence of a variable

[Thomas Scheffler]

Hi,

I have several xslt documents that import each other in a specific way
to allow resulting documents to be layout in a common way. In the
"master stylesheet" I need some kind of test to check if a variable exists.
<xsl:if test="$testvariable"> is false when the variable exists. Is
there any test that delivers "true" if the variable exists and return
"false" if not instead of an error message saying that the variable is
not defined? I hope somebody could help me on this issue.

Thank you in advance

Thomas Scheffler