attempt to fix cysouw#15 to maintain beamer frame attributes

add "restart" class instead of overwriting other classes
only remove inserted "restart" class

pandoc-ling.lua

@@ -216,7 +216,7 @@ function addDivToHeader (head)
       and head.level == 1 
       and head.classes[1] ~= "unnumbered"
   then
-    head.attr = {id = head.identifier, class = "restart"}
+    head.classes:extend({"restart"})
     return pandoc.Div(head)
   end
 end
@@ -229,10 +229,16 @@ function processDiv (div)
 
   -- keep track of chapters (header == 1)
   -- included in this loop by trick "addDivToHeader"
-  if div.content[1].tag == "Header" and div.content[1].classes[1] == "restart"  then
+  if div.content[1].tag == "Header" then
     chapter = chapter + 1
     counterInChapter = 0
-    div.content[1].attr = {id = div.content[1].identifier, class = ""}
+    classes = {}
+    for  _,cls in ipairs(div.content[1].classes) do
+        if cls ~= "restart" then
+            table.insert(classes, cls)
+        end
+    end
+    div.content[1].classes = classes
     -- remove div
     return div.content
   end