Fix for issue #2828: require a minumum of one value for stage/@type.

Author: martindholmes

P5/Source/Specs/stage.xml

@@ -41,7 +41,7 @@
       <desc versionDate="2007-05-04" xml:lang="es">indica el tipo de acotación.</desc>
       <desc versionDate="2007-01-21" xml:lang="it">indica il tipo di direttiva di scena</desc>
       <desc versionDate="2017-06-13" xml:lang="de">beschreibt die Art der Regieanweisung.</desc>
-      <datatype maxOccurs="unbounded" minOccurs="0"><dataRef key="teidata.enumerated"/></datatype>
+      <datatype maxOccurs="unbounded" minOccurs="1"><dataRef key="teidata.enumerated"/></datatype>
       <valList type="semi">
         <valItem ident="setting">
           <desc versionDate="2007-06-27" xml:lang="en">describes a setting.</desc>