> How to clarify the error message? What about (borrowing 
> language from the above paragraph):
> "Invalid initial / or // in path step: Cannot select the root 
> node of the tree that contains the context node, because the 
> context item is not a node."
> 
> It's a little long, but given that I'm not the first one who 
> has been unhelped by the existing error message, wouldn't it 
> be worth it to make this somewhat obscure problem clearer?

I'll see what I can do to improve it.

One of the difficulties in designing run-time error messages is that it's
dangerous to make assumptions about what constructs were used in the
original source code, since the same run-time code can be produced from many
different source code formulations. However, there are many cases where
Saxon saves information at compile time for use in the event of a run-time
error, so that better diagnostics can be produced, and it might be possible
to do the same here.

Michael Kay
http://www.saxonica.com/