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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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.