Container element for one set of keywords (<kwd>s) used to describe a document.
The following, in order:
<label> Label (of an Equation, Figure, Reference, etc.), zero or one
<title> Title, zero or one
Any one of:
One or more of any of:
<kwd> Keyword
<compound-kwd> Compound Keyword
<x> X - Generated Text and Punctuation
<unstructured-kwd-group> Unstructured Keyword Group