Error XTDE0070 reads:

When the focus is undefined, evaluation of any expression that references the context item, context position, or context size results in a non-recoverable dynamic error.

This error is already defined in XPath. We don't need to change the error code. The general principle is that errors occuring during the evaluation of an XPath expression retain their XPath error code (there are exceptions where we want to make the error more specific, but in this case we aren't adding value in that way.)

Suggest deleting XTDE0070 and replacing it with a reference to the XPath error.