Update spec; allow internal delimiter runs to match if...

both have lengths that are multiples of 3.

See commonmark/commonmark-spec#528.

src/inlines.c

@@ -642,7 +642,8 @@ static void process_emphasis(subject *subj, delimiter *stack_bottom) {
           // interior closer of size 2 can't match opener of size 1
           // or of size 1 can't match 2
           if (!(closer->can_open || opener->can_close) ||
-              ((opener->length + closer->length) % 3) != 0) {
+	      closer->length % 3 == 0 ||
+              (opener->length + closer->length) % 3 != 0) {
             opener_found = true;
             break;
           }

test/spec.txt

@@ -1922,9 +1922,11 @@ bar
 
 
 An [info string] can be provided after the opening code fence.
-Opening and closing spaces will be stripped, and the first word, prefixed
-with `language-`, is used as the value for the `class` attribute of the
-`code` element within the enclosing `pre` element.
+Although this spec doesn't mandate any particular treatment of
+the info string, the first word is typically used to specify
+the language of the code block. In HTML output, the language is
+normally indicated by adding a class to the `code` element consisting
+of `language-` followed by the language name.
 
 ```````````````````````````````` example
 ```ruby
@@ -6107,7 +6109,8 @@ The following rules define emphasis and strong emphasis:
     [delimiter runs].  If one of the delimiters can both
     open and close emphasis, then the sum of the lengths of the
     delimiter runs containing the opening and closing delimiters
-    must not be a multiple of 3.
+    must not be a multiple of 3 unless both lengths are
+    multiples of 3.
 
 10. Strong emphasis begins with a delimiter that
     [can open strong emphasis] and ends with a delimiter that
@@ -6117,7 +6120,8 @@ The following rules define emphasis and strong emphasis:
     [delimiter runs].  If one of the delimiters can both open
     and close strong emphasis, then the sum of the lengths of
     the delimiter runs containing the opening and closing
-    delimiters must not be a multiple of 3.
+    delimiters must not be a multiple of 3 unless both lengths
+    are multiples of 3.
 
 11. A literal `*` character cannot occur at the beginning or end of
     `*`-delimited emphasis or `**`-delimited strong emphasis, unless it
@@ -6736,7 +6740,8 @@ is precluded by the condition that a delimiter that
 can both open and close (like the `*` after `foo`)
 cannot form emphasis if the sum of the lengths of
 the delimiter runs containing the opening and
-closing delimiters is a multiple of 3.
+closing delimiters is a multiple of 3 unless
+both lengths are multiples of 3.
 
 
 For the same reason, we don't get two consecutive
@@ -6776,6 +6781,23 @@ omitted:
 ````````````````````````````````
 
 
+When the lengths of the interior closing and opening
+delimiter runs are *both* multiples of 3, though,
+they can match to create emphasis:
+
+```````````````````````````````` example
+foo***bar***baz
+.
+<p>foo<em><strong>bar</strong></em>baz</p>
+````````````````````````````````
+
+```````````````````````````````` example
+foo******bar*********baz
+.
+<p>foo<strong><strong><strong>bar</strong></strong></strong>***baz</p>
+````````````````````````````````
+
+
 Indefinite levels of nesting are possible:
 
 ```````````````````````````````` example