treat pervasive "Interrupt" constructor as illegal -- superseded by Exn.Interrupt for internal use and Exn.is_interrupt/Exn.interrupt in user-space;
--- a/src/Pure/General/exn.ML Fri Nov 12 10:58:09 2010 +0100
+++ b/src/Pure/General/exn.ML Fri Nov 12 11:36:01 2010 +0100
@@ -85,3 +85,6 @@
handle EXCEPTIONS (exn :: _) => reraise exn;
end;
+
+datatype illegal = Interrupt;
+