foo`bar because a lone backtick in the body cannot * form a valid two-backtick closer. Combined with the edge-space * strip rule, you can embed backticks right at the boundaries: the * input `` `foo` `` renders as `foo`. * * Extends GfmBacktickSingle to inherit handle() and normalizeBody; * only the delimiter length and the body character class differ. * Sort and category match the parent so the two modes share one * precedence slot — the (?