Markdown reader: Add a syntax for resetting example list counter.

This is needed often at the beginning of a chapter.

Closes #10940.

MANUAL.txt

@@ -4726,6 +4726,17 @@ This only works reliably, though, if the repeated item is in a list
 by itself, because each numbered example list will be numbered
 continuously from its starting number.
 
+In a longer (book-length) work, one might need to reset the example
+list counter at the beginning of each chapter. One can do that using the
+following syntax:
+
+    (1@foo) This item will be numbered 1, and subsequent example
+        list items will be numbered 2, 3, etc.
+
+Any number may be used instead of `1`. Note that if there are several
+example list items in a row, the number only has an effect if placed
+on the first of them.
+
 ### Ending a list ###
 
 What if you want to put an indented code block after a list?

src/Text/Pandoc/Parsing/Lists.hs

@@ -112,12 +112,16 @@ decimal = do
 exampleNum :: (Stream s m Char, UpdateSourcePos s Char)
            => ParsecT s ParserState m (ListNumberStyle, Int)
 exampleNum = do
+  mbNum <- safeRead . T.pack <$> many digit
   char '@'
   lab <- T.pack . concat <$>
                     many (many1 alphaNum <|>
                           try (do c <- char '_' <|> char '-'
                                   cs <- many1 alphaNum
                                   return (c:cs)))
+  case mbNum of
+      Nothing -> pure ()
+      Just n -> updateState $ \s -> s{ stateNextExample = n }
   st <- getState
   case M.lookup lab (stateExamples st) of
     Nothing -> do -- new label

test/command/10940.md

@@ -0,0 +1,27 @@
+```
+% pandoc -t plain
+# Chapter one
+
+(@foo) an example list.
+(@) second item.
+
+# Chapter two
+
+(1@bar) reset the counter to one.
+(@) another.
+
+Foo is (@foo) and bar is (@bar).
+^D
+Chapter one
+
+(1) an example list.
+(2) second item.
+
+Chapter two
+
+(1) reset the counter to one.
+(2) another.
+
+Foo is (1) and bar is (1).
+
+```