Strip more unneeded markup

filter-clean_ccel.lua

@@ -21,7 +21,7 @@ Span = function (element)
 end
 
 Div = function (element)
-  if (#element.classes == 1 and hasClass(element, { "mnote" })) then
+  if (#element.classes == 1 and hasClass(element, { "mnote", "book-content" })) then
     return element.content
   end
   return element
@@ -30,3 +30,12 @@ end
 LineBreak = function ()
   return {}
 end
+
+Header = function (element)
+  -- element.attr = {}
+  return pandoc.Header(element.level, element.content)
+end
+
+HorizontalRule = function ()
+  return {}
+end