src/Pure/General/pretty.ML

 fun make_block {markup, consistent, indent} body =
   let
     val indent' = force_nat indent;
     fun body_length prts len =
       let
-        val (line, rest) = take_prefix (fn Break (true, _, _) => false | _ => true) prts;
+        val (line, rest) = chop_prefix (fn Break (true, _, _) => false | _ => true) prts;
         val len' = Int.max (fold (Integer.add o length) line 0, len);
       in
         (case rest of
           Break (true, _, ind) :: rest' =>
             body_length (Break (false, indent' + ind, 0) :: rest') len'