Suppose, for instance, that a Python programmer wants to write a script that automatically sends a message to her cell phone when the bus she takes to work rounds a particular corner.

The xsl:message element does not change the result tree produced by the XSLT stylesheet at all. It merely outputs a message somewhere the programmer can see it.

While this is a step in the right direction, it's still a pretty useless message in terms of telling the user or a programmer working on the application what actually went wrong.