diff --git a/src/Pure/General/symbol_explode.ML b/src/Pure/General/symbol_explode.ML
--- a/src/Pure/General/symbol_explode.ML
+++ b/src/Pure/General/symbol_explode.ML
@@ -1,53 +1,52 @@
 (*  Title:      Pure/General/symbol_explode.ML
     Author:     Makarius
 
 String explode operation for Isabelle symbols (see also symbol.ML).
 *)
 
 structure Symbol: sig val explode: string -> string list end =
 struct
 
 fun is_ascii_letter c = #"A" <= c andalso c <= #"Z" orelse #"a" <= c andalso c <= #"z";
 fun is_ascii_letdig c =
   is_ascii_letter c orelse #"0" <= c andalso c <= #"9" orelse c = #"_" orelse c = #"'";
 
 fun is_utf8 c = c >= #"\128";
 fun is_utf8_trailer c = #"\128" <= c andalso c < #"\192";
 fun is_utf8_control c = #"\128" <= c andalso c < #"\160";
 
 fun explode string =
   let
     fun char i = String.sub (string, i);
-    fun string_range i j = String.substring (string, i, j - i);
+    fun substring i j = String.substring (string, i, j - i);
 
     val n = size string;
     fun test pred i = i < n andalso pred (char i);
     fun many pred i = if test pred i then many pred (i + 1) else i;
-    fun maybe pred i = if test pred i then i + 1 else i;
-    fun maybe_char c = maybe (fn c' => c = c');
+    fun maybe_char c i = if test (fn c' => c = c') i then i + 1 else i;
     fun maybe_ascii_id i = if test is_ascii_letter i then many is_ascii_letdig (i + 1) else i;
 
     fun scan i =
       if i < n then
         let val ch = char i in
           (*encoded newline*)
           if ch = #"\^M" then "\n" :: scan (maybe_char #"\n" (i + 1))
           (*pseudo utf8: encoded ascii control*)
           else if ch = #"\192" andalso
             test is_utf8_control (i + 1) andalso not (test is_utf8 (i + 2))
           then chr (Char.ord (char (i + 1)) - 128) :: scan (i + 2)
           (*utf8*)
           else if is_utf8 ch then
             let val j = many is_utf8_trailer (i + 1)
-            in string_range i j :: scan j end
+            in substring i j :: scan j end
           (*named symbol*)
           else if ch = #"\\" andalso test (fn c => c = #"<") (i + 1) then
             let val j = (i + 2) |> maybe_char #"^" |> maybe_ascii_id |> maybe_char #">"
-            in string_range i j :: scan j end
+            in substring i j :: scan j end
           (*single character*)
           else String.str ch :: scan (i + 1)
         end
       else [];
   in scan 0 end;
 
 end;