Unknoww Scheme expression

What's up when xpointer expression contains unknown schemes ?

When multiple pointer parts are provided, an XPointer processor must evaluate them in left-to-right order. If the XPointer processor does not support the scheme used in a pointer part, it skips that pointer part. If a pointer part does not identify any subresources, evaluation continues and the next pointer part, if any, is evaluated. The result of the first pointer part whose evaluation identifies one or more subresources is reported by the XPointer processor as the result of the pointer as a whole, and evaluation stops. If no pointer part identifies subresources, it is an error.

Example

Given a simple xml :

            <catalog>
                <book id="book1">
                    <author id="auth1">Gambardella, Matthew</author>
                    <title>XML Developer's Guide</title>
                    <genre id="bk101">Computer</genre>
                    <price>44.95</price>
                    <publish_date>2000-10-01</publish_date>
                    <description>An in-depth look at creating applications
                        with XML.</description>
                </book>
            </catalog>

Expressions matching nodes:

Pointer content	Result	ErrorHandler
xlimit(1)element(auth1)	<author id="auth1">Gambardella, Matthew</author>	Warning: 'xlimit' scheme is not supported
xlimit(1)		Warning: 'xlimit' scheme is not supported Unknown pointer expression