replaced find_substring by first_field;

--- a/src/Pure/General/yxml.ML	Wed Aug 27 17:54:31 2008 +0200
+++ b/src/Pure/General/yxml.ML	Wed Aug 27 20:36:23 2008 +0200
@@ -101,10 +101,10 @@
 (* parsing *)
 
 fun parse_attrib s =
-  (case find_substring "=" s of
+  (case first_field "=" s of
     NONE => err_attribute ()
-  | SOME 0 => err_attribute ()
-  | SOME i => (String.substring (s, 0, i), String.extract (s, i + 1, NONE)));
+  | SOME ("", _) => err_attribute ()
+  | SOME att => att);
 
 fun parse_chunk ["", ""] = pop
   | parse_chunk ("" :: name :: atts) = push name (map parse_attrib atts)

--- a/src/Pure/library.ML	Wed Aug 27 17:54:31 2008 +0200
+++ b/src/Pure/library.ML	Wed Aug 27 20:36:23 2008 +0200
@@ -142,7 +142,7 @@
   val fold_string: (string -> 'a -> 'a) -> string -> 'a -> 'a
   val exists_string: (string -> bool) -> string -> bool
   val forall_string: (string -> bool) -> string -> bool
-  val find_substring: string -> string -> int option
+  val first_field: string -> string -> (string * string) option
   val enclose: string -> string -> string -> string
   val unenclose: string -> string
   val quote: string -> string
@@ -732,15 +732,19 @@
 
 fun forall_string pred = not o exists_string (not o pred);
 
-fun find_substring s str =
+fun first_field sep str =
   let
-    val n = size s;
+    val n = size sep;
     val len = size str;
     fun find i =
       if i + n > len then NONE
-      else if String.substring (str, i, n) = s then SOME i
+      else if String.substring (str, i, n) = sep then SOME i
       else find (i + 1);
-  in find 0 end;
+  in
+    (case find 0 of
+      NONE => NONE
+    | SOME i => SOME (String.substring (str, 0, i), String.extract (str, i + n, NONE)))
+  end;
 
 (*enclose in brackets*)
 fun enclose lpar rpar str = lpar ^ str ^ rpar;