Docx Reader: preprocess Document body to unwrap "w:sdt" elements

We walk through the document (using the zipper in
Text.XML.Light.Cursor) to unwrap the sdt tags before doing the rest of
the parsing of the document. Note that the function is generically
named `walkDocument` in case we need to do any further preprocessing
in the future.

Closes #4190

src/Text/Pandoc/Readers/Docx/Parse.hs

@@ -73,6 +73,7 @@ import Text.TeXMath (Exp)
 import Text.TeXMath.Readers.OMML (readOMML)
 import Text.TeXMath.Unicode.Fonts (Font (..), getUnicode, stringToFont)
 import Text.XML.Light
+import qualified Text.XML.Light.Cursor as XMLC
 
 data ReaderEnv = ReaderEnv { envNotes         :: Notes
                            , envComments      :: Comments
@@ -117,6 +118,32 @@ mapD f xs =
   in
    concatMapM handler xs
 
+unwrapSDT :: NameSpaces -> Content -> Content
+unwrapSDT ns (Elem element)
+  | isElem ns "w" "sdt" element
+  , Just sdtContent <- findChildByName ns "w" "sdtContent" element
+  , child : _    <- elChildren sdtContent
+  = Elem child
+unwrapSDT _ content = content
+
+walkDocument' :: NameSpaces -> XMLC.Cursor -> XMLC.Cursor
+walkDocument' ns cur =
+  let modifiedCur = XMLC.modifyContent (unwrapSDT ns) cur
+  in
+    case XMLC.nextDF modifiedCur of
+      Just cur' -> walkDocument' ns cur'
+      Nothing   -> XMLC.root modifiedCur
+
+walkDocument :: NameSpaces -> Element -> Maybe Element
+walkDocument ns element =
+  let cur = XMLC.fromContent (Elem element)
+      cur' = walkDocument' ns cur
+  in
+    case XMLC.toTree cur' of
+      Elem element' -> Just element'
+      _             -> Nothing
+
+
 data Docx = Docx Document
           deriving Show
 
@@ -298,7 +325,10 @@ archiveToDocument zf = do
   docElem <- maybeToD $ (parseXMLDoc . UTF8.toStringLazy . fromEntry) entry
   let namespaces = elemToNameSpaces docElem
   bodyElem <- maybeToD $ findChildByName namespaces "w" "body" docElem
-  body <- elemToBody namespaces bodyElem
+  let bodyElem' = case walkDocument namespaces bodyElem of
+        Just e -> e
+        Nothing -> bodyElem
+  body <- elemToBody namespaces bodyElem'
   return $ Document namespaces body
 
 elemToBody :: NameSpaces -> Element -> D Body