In Extended Backus-Naur Form (EBNF) there is one form called a "special sequence" which is surrounded by questions marks ?...?
What does this mean?
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Tyler Durden
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Quoting from the Wikipedia page:
According to the ISO 14977 standard EBNF is meant to be extensible, and two facilities are mentioned. The first is part of EBNF grammar, the special sequence, which is arbitrary text enclosed with question marks. The interpretation of the text inside a special sequence is beyond the scope of the EBNF standard.
Yuval Filmus
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IOW, it's a description in some human-readable language which explains the set of derivations of a non-terminal, to be used in cases where the EBNF formalism is not powerful or convenient enough. – rici Jul 31 '16 at 04:43