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Symbols.
This is a sum-of-products (i.e., union) type, introduced by fty::defflexsum.
As explained in [RFC:2.3], the terminal values of ABNF are natural numbers.
Rule names are the nonterminal symbols of ABNF.
In analogy with the typical notion of formal grammar in textbooks, an ABNF symbol is a terminal or a nonterminal, i.e. a natural number or a rule name.
Since natural numbers and rule names are disjoint, we put them together without tags.