On the specification page for an element is a field labeled "May
contain". The value of the field is sometimes "Empty element".
<ptr> is an empty element. That makes sense.
But so too is <gi> and <datatype>, even though neither is, in XML's
defined use of the term, an "empty element."
Is "empty element" an idiosyncratic TEI term, meaning, I guess, "empty
of further TEI elements"? Or is its appearance in, say, <gi> and
<datatype> bugs in the documentation?