TMRM: isa/sub - irreflexive relation vs. reflexive relation

Hi there,

Johannes' question [1] brought me to another question: Why is the
"isa" relation irreflexive by definition and the "sub" relation
reflexive by definition (c.f. TMRM 5 a) and 5 b)?

I understand why the "sub" relation is reflexive (implicitly since
every proxy is a subclass of itself by definition), but I wonder why
stating "x isa x" is explicitly forbidden.

What's the rationale behind this design decision?

[1] <http://www.infoloom.com/pipermail/topicmapmail/2011q4/009161.html>

Thanks and best regards,
Lars
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