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In general, a given witness should only be referred to once within an <app> (either on the @wit of <rdg> or of <lem>).

This snippet of ODD code may be included in the <elementSpec> for app to test the collection of pointers to witnesses within a given <app> for duplicates.

This test is insufficient for a variety of reasons. First, the TEI Guidelines explicit permit the specification of witnesses (on @wit) for one reading to be omitted. (See 12.2.3 The Parallel Segmentation Method.)

Second, this code only tests the string value of a given pointer, not what it points at. Thus the value of @xml:base is not taken into account, and the values "witnesses.xml#A" & "./witnesses.xml#A" will incorrectly be considered as different. Similarly, this code will consider it OK when two @wit pointers point two two different witness elements that list the same MSS. (Arguably bad encoding, but not technically incorrect.)

This also means that a repeated pointer to a witness (also bad encoding, but not technically incorrect) will be flagged as an error.

And, perhaps most importantly, it means that this test does not check that the pointers on @wit actually point to witnesses, as opposed to, say, persons.

<constraintSpec ident="unique-witness-within-app" scheme="isoschematron">
    <sch:let name="wits" value="tokenize( normalize-space( string-join( .//@wit,' ') ),' ')"/>
    <sch:let name="witless" value="distinct-values( $wits )"/>
    <sch:report test="count( $wits ) ne count( $witless )">Duplicate witness within 'app'</sch:report>
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