xml comment syntax

Well, really of little importance, but I can't determine if this is really
an error:

  <!--- Replaced so that we have a unionOf construct
        Aug 20 02 MKS, suggestion by R. Volz.

  <owl:Class rdf:ID="Fruit">
    <rdfs:subClassOf rdf:resource="#EdibleThing" />
  </owl:Class>

  -->

as reported while running thea2 unit tests:

Warning: Unknown file-extension: . Assuming RDF/XML

ERROR: SGML2PL(xmlns):
http://www.w3.org/TR/2003/PR-owl-guide-20031209/food:869: Illegal comment,
found "<!---"

% Parsed "http://www.w3.org/TR/2003/PR-owl-guide-20031215/food" in 0.06
sec; 954 triples


wikipedia <http://en.wikipedia.org/wiki/XML#Comments> informal description
seems suggest that should be valid...
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