clarified signature, roughly following Isabelle/Scala;

--- a/src/Pure/PIDE/xml.ML	Tue Sep 10 19:57:45 2024 +0200
+++ b/src/Pure/PIDE/xml.ML	Tue Sep 10 20:06:51 2024 +0200
@@ -37,7 +37,7 @@
   val is_empty_body: body -> bool
   val string: string -> body
   val enclose: string -> string -> body -> body
-  val add_content: tree -> Buffer.T -> Buffer.T
+  val traverse_text: (string -> 'a -> 'a) -> XML.tree -> 'a -> 'a
   val trim_blanks: body -> body
   val filter_elements: {remove: string -> bool, expose: string -> bool} -> body -> body
   val header: string
@@ -93,15 +93,15 @@
 fun enclose bg en body = string bg @ body @ string en;
 
 
-(* text content *)
+(* traverse text *)
 
-fun add_content tree =
+fun traverse_text f tree =
   (case unwrap_elem_body tree of
-    SOME ts => fold add_content ts
+    SOME ts => fold (traverse_text f) ts
   | NONE =>
       (case tree of
-        Elem (_, ts) => fold add_content ts
-      | Text s => Buffer.add s));
+        Elem (_, ts) => fold (traverse_text f) ts
+      | Text s => f s));
 
 
 (* trim blanks *)

--- a/src/Tools/Code/code_printer.ML	Tue Sep 10 19:57:45 2024 +0200
+++ b/src/Tools/Code/code_printer.ML	Tue Sep 10 20:06:51 2024 +0200
@@ -138,8 +138,10 @@
 fun markup_stmt sym =
   Pretty.mark (code_presentationN, [(stmt_nameN, Code_Symbol.marker sym)]);
 
+val add_text = XML.traverse_text Buffer.add;
+
 fun filter_presentation [] xml =
-      Buffer.build (fold XML.add_content xml)
+      Buffer.build (fold add_text xml)
   | filter_presentation presentation_syms xml =
       let
         val presentation_idents = map Code_Symbol.marker presentation_syms
@@ -149,7 +151,7 @@
         fun add_content_with_space tree (is_first, buf) =
           buf
           |> not is_first ? Buffer.add "\n\n"
-          |> XML.add_content tree
+          |> add_text tree
           |> pair false;
         fun filter (XML.Elem (name_attrs, xs)) =
               fold (if is_selected name_attrs then add_content_with_space else filter) xs