Specifying how to specify a BNF-like grammar in a BNF-like grammar

I love this bit of the XML specification, the part where in definitions 47-50, it states how to use * to mean “the preceding can appear zero or more times” and + to mean “the preceding can appear one or more times” and ? to mean “the preceding is optional” and | to mean “either of these may appear”, but it does this using those very same symbols, with the same meanings.

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