XSLT: Poor browser compilation reporting.

You have to love the lack of context with web browser XSLT processing:

Firefox - "Error during XSLT transformation: Evaluating an invalid expression."

All down to a double equals in an expression.  Something I regularly make the mistake of doing, but under normal cercumstances, is easy to spot via xsltproc:

XPath error : Invalid expression
$leftspan == 2
           ^


compilation error: file xxx-xxxxx.xsl line 193 element if
xsl:if : could not compile test expression '$leftspan == 2'