Element date as part of watermark element

@djbpitt Right now the Schematron rule states:

In an album reference in a watermark, the num and date elements must alternate, starting with a num.

But when in the description I have no date but just the number in watermark album and I cannot check it the markup will be:

<watermark>
  <re:motif>Anchor in circle with anchor</re:motif>
    <ref type="bibl" target="bib:Moshin1973">
       <num>1942</num>
       <date/>
         </ref>
         </watermark>

In Zograf library I encoded the informaton like that:

<watermark>
<motif xmlns="http://www.ilit.bas.bg/repertorium/ns/3.0" facs="http://memoryofpaper.oeaw.ac.at/mosin/mosin.php?wmid=3050">Камбана в кръг</motif> <term>много подобен</term> <ref target="#Mošin1957" type="bibl">Mošin, Traljić 1957</ref><num>3050</num><date type="watermark" notBefore="1355" notAfter="1359">1355–1359</date>
</watermark>

And when I have no information about the date I just not use the element. Do we need an empty date element in this place. How we should proceed?

djbpitt / repertorium

Element date as part of watermark element #20