Elements consist of a start tag (e.g. "<start>"), its contents, and an end tag (e.g. "</start>").

The contents of an element are ordered.

Start tags can have attributes -- name/value pairs (e.g. "<start kind="slow"/>).

Elements must be well-formed: balanced, conforming syntax, attributes are valid, no duplicates, etc.

<top>
   <next>
      <bottom>   ...  </bottom>
   </next>
</top>

(more will be said about well-formed later).

The element name must conform to:

[5]    Name    ::=    (Letter | '_' | ':') (NameChar)*