This stuff is not worth worrying about. This rule was inherited from SGML; its inclusion in SGML was actually a design error. This was retained in XML not only for compatibility with SGML (not quite a good enough reason; I voted against it) but because some of the most popular existing SGML tools actually rely on it for certain internal optimizations.
It's likely that quite a few XML products will never bother checking for violations of this rule, because it's hard; if you're writing a DTD and you get a complaint about a nondeterministic content model, then you might find it worthwhile to read the appendix.
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