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
<title> Title, zero or one
One or more of any of:
<kwd> Keyword
<compound-kwd> Compound Keyword
<nested-kwd> Nested Keyword