Don't generate empty H1 after hrule slide breaks.

We now use a slide-level header with contents [Str "\0"] to mark
an hrule break.  This avoids creation of an empty H1 in these
contexts.

Closes #484.

src/Text/Pandoc/Slides.hs

@@ -49,9 +49,10 @@ prepSlides :: Int -> [Block] -> [Block]
 prepSlides slideLevel = ensureStartWithH . splitHrule
   where splitHrule (HorizontalRule : Header n xs : ys)
                        | n == slideLevel = Header slideLevel xs : splitHrule ys
-        splitHrule (HorizontalRule : xs) = Header slideLevel [] : splitHrule xs
+        splitHrule (HorizontalRule : xs) = Header slideLevel [Str "\0"] :
+                                           splitHrule xs
         splitHrule (x : xs)              = x : splitHrule xs
         splitHrule []                    = []
         ensureStartWithH bs@(Header n _:_)
                        | n <= slideLevel = bs
-        ensureStartWithH bs              = Header slideLevel [] : bs
+        ensureStartWithH bs              = Header slideLevel [Str "\0"] : bs

src/Text/Pandoc/Writers/HTML.hs

@@ -262,7 +262,9 @@ elementToHtml slideLevel opts (Sec level num id' title' elements) = do
   -- always use level 1 for slide titles
   let level' = if slide then 1 else level
   let titleSlide = slide && level < slideLevel
-  header' <- blockToHtml opts (Header level' title')
+  header' <- if title' == [Str "\0"]  -- marker for hrule
+                then return mempty
+                else blockToHtml opts (Header level' title')
   let isSec (Sec _ _ _ _ _) = True
       isSec (Blk _)         = False
   innerContents <- mapM (elementToHtml slideLevel opts)

src/Text/Pandoc/Writers/LaTeX.hs

@@ -243,8 +243,11 @@ elementToBeamer slideLevel  (Sec lvl _num _ident tit elts)
       let fragile = if not $ null $ queryWith hasCodeBlock elts ++ queryWith hasCode elts
                        then "[fragile]"
                        else ""
-      let slideStart = Para $ RawInline "latex" ("\\begin{frame}" ++ fragile ++
+      let slideStart = Para $ RawInline "latex" ("\\begin{frame}" ++ fragile) :
-                "\\frametitle{") : tit ++ [RawInline "latex" "}"]
+                if tit == [Str "\0"]  -- marker for hrule
+                   then []
+                   else (RawInline "latex" "\\frametitle{") : tit ++
+                        [RawInline "latex" "}"]
       let slideEnd = RawBlock "latex" "\\end{frame}"
       -- now carve up slide into blocks if there are sections inside
       bs <- concat `fmap` mapM (elementToBeamer slideLevel) elts