This patch handles the following two rules
1) A delimiter run is either (a) not followed by a Unicode punctuation
character, or (b) followed by a Unicode punctuation character and
preceded by Unicode whitespace or a Unicode punctuation character.
2) A _ can be used to open/close a delimiter run if it's (a) not part
of a left-flanking delimiter run or (b) part of a left-flanking
delimiter run followed by a Unicode punctuation character.
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