added is_printable: string -> bool;

--- a/src/Pure/library.ML	Mon Nov 18 17:27:59 1996 +0100
+++ b/src/Pure/library.ML	Mon Nov 18 17:28:19 1996 +0100
@@ -374,6 +374,9 @@
 
 val is_letdig = is_quasi_letter orf is_digit;
 
+(*printable chars*)
+fun is_printable c = ord c > ord " " andalso ord c <= ord "~";
+
 
 (*lower all chars of string*)
 val to_lower =