Spec changes in strong/emph parsing.

See
https://talk.commonmark.org/t/emphasis-strong-emphasis-corner-cases/2123
for motivation.

This restores intuitive parsings for a number of changes.

The main change is to disallow strong or emph when one of
the delimiters is "internal" and the sum of the lengths of
the enclosing delimiter runs is a multiple of 3.

Thus,

    **foo*bar***

gets parsed `<strong>foo*bar</strong>` rather than
`<em><em>foo</em>bar</em>**` as before.

1 files changed, 38 insertions, 35 deletions

diff --git a/spec.txt b/spec.txt
index 99365cd..c7b1fcd 100644
--- a/spec.txt
+++ b/spec.txt
@@ -5964,18 +5964,22 @@ The following rules define emphasis and strong emphasis:
 
 9.  Emphasis begins with a delimiter that [can open emphasis] and ends
     with a delimiter that [can close emphasis], and that uses the same
-    character (`_` or `*`) as the opening delimiter.  There must
-    be a nonempty sequence of inlines between the open delimiter
-    and the closing delimiter; these form the contents of the emphasis
-    inline.
+    character (`_` or `*`) as the opening delimiter.  The
+    opening and closing delimiters must belong to separate
+    [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.
 
 10. Strong emphasis begins with a delimiter that
     [can open strong emphasis] and ends with a delimiter that
     [can close strong emphasis], and that uses the same character
-    (`_` or `*`) as the opening delimiter.
-    There must be a nonempty sequence of inlines between the open
-    delimiter and the closing delimiter; these form the contents of
-    the strong emphasis inline.
+    (`_` or `*`) as the opening delimiter.  The
+    opening and closing delimiters must belong to separate
+    [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.
 
 11. A literal `*` character cannot occur at the beginning or end of
     `*`-delimited emphasis or `**`-delimited strong emphasis, unless it
@@ -5999,9 +6003,7 @@ the following principles resolve ambiguity:
     so that the second begins before the first ends and ends after
     the first ends, the first takes precedence. Thus, for example,
     `*foo _bar* baz_` is parsed as `<em>foo _bar</em> baz_` rather
-    than `*foo <em>bar* baz</em>`.  For the same reason,
-    `**foo*bar**` is parsed as `<em><em>foo</em>bar</em>*`
-    rather than `<strong>foo*bar</strong>`.
+    than `*foo <em>bar* baz</em>`.
 
 16. When there are two potential emphasis or strong emphasis spans
     with the same closing delimiter, the shorter one (the one that
@@ -6581,18 +6583,30 @@ __foo_ bar_
 <p><em>foo <strong>bar</strong> baz</em></p>
 ````````````````````````````````
 
-
-But note:
-
 ```````````````````````````````` example
 *foo**bar**baz*
 .
-<p><em>foo</em><em>bar</em><em>baz</em></p>
+<p><em>foo<strong>bar</strong>baz</em></p>
 ````````````````````````````````
 
+Note that in the preceding case, the interpretation
+
+``` markdown
+<p><em>foo</em><em>bar<em></em>baz</em></p>
+```
+
+
+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.
+
+The same condition ensures that the following
+cases are all strong emphasis nested inside
+emphasis, even when the interior spaces are
+omitted:
 
-The difference is that in the preceding case, the internal delimiters
-[can close emphasis], while in the cases with spaces, they cannot.
 
 ```````````````````````````````` example
 ***foo** bar*
@@ -6608,17 +6622,18 @@ The difference is that in the preceding case, the internal delimiters
 ````````````````````````````````
 
 
-Note, however, that in the following case we get no strong
-emphasis, because the opening delimiter is closed by the first
-`*` before `bar`:
-
 ```````````````````````````````` example
 *foo**bar***
 .
-<p><em>foo</em><em>bar</em>**</p>
+<p><em>foo<strong>bar</strong></em></p>
 ````````````````````````````````
 
 
+```````````````````````````````` example
+*foo**bar***
+.
+<p><em>foo<strong>bar</strong></em></p>
+````````````````````````````````
 
 Indefinite levels of nesting are possible:
 
@@ -6712,18 +6727,13 @@ ____foo__ bar__
 ````````````````````````````````
 
 
-But note:
-
 ```````````````````````````````` example
 **foo*bar*baz**
 .
-<p><em><em>foo</em>bar</em>baz**</p>
+<p><strong>foo<em>bar</em>baz</strong></p>
 ````````````````````````````````
 
 
-The difference is that in the preceding case, the internal delimiters
-[can close emphasis], while in the cases with spaces, they cannot.
-
 ```````````````````````````````` example
 ***foo* bar**
 .
@@ -7039,13 +7049,6 @@ Rule 15:
 
 
 ```````````````````````````````` example
-**foo*bar**
-.
-<p><em><em>foo</em>bar</em>*</p>
-````````````````````````````````
-
-
-```````````````````````````````` example
 *foo __bar *baz bim__ bam*
 .
 <p><em>foo <strong>bar *baz bim</strong> bam</em></p>