Jan-22-2018, 03:50 AM
It's a metasyntax notation called extended Backus-Naur form (EBNF).
The BNF uses the symbols (<, >, |, ::=) for itself, but does not include quotes around terminal strings. This prevents these characters from being used in the languages, and requires a special symbol for the empty string. In EBNF, terminals are strictly enclosed within quotation marks ("..." or '...'). The angle brackets ("<...>") for nonterminals can be omitted.
see: https://www.w3.org/TR/hdml20-6.html#HEADING6-3
The BNF uses the symbols (<, >, |, ::=) for itself, but does not include quotes around terminal strings. This prevents these characters from being used in the languages, and requires a special symbol for the empty string. In EBNF, terminals are strictly enclosed within quotation marks ("..." or '...'). The angle brackets ("<...>") for nonterminals can be omitted.
see: https://www.w3.org/TR/hdml20-6.html#HEADING6-3