Container element for one set of keywords (such as <kwd>s, <nested-kwd>s, or <compound-kwd>s) used to describe a document.
The following, in order:
<label> Label (of an Equation, Figure, Reference, etc.), zero or one
Any one of:
One or more of any of:
<unstructured-kwd-group> Unstructured Keyword Group, zero or more