adhoc fix of is_blank;

--- a/src/Pure/Syntax/symbol.ML	Tue Mar 10 13:27:13 1998 +0100
+++ b/src/Pure/Syntax/symbol.ML	Tue Mar 10 14:27:44 1998 +0100
@@ -183,7 +183,8 @@
   | is_quasi_letter s = is_letter s;
 
 val is_blank =
-  fn " " => true | "\t" => true | "\n" => true | "\^L" => true | "\\<space2>" => true
+  fn " " => true | "\t" => true | "\n" => true | "\^L" => true
+    | "\160" => true | "\\<space2>" => true
     | _ => false;
 
 val is_letdig = is_quasi_letter orf is_digit;