Exception matcher in Specs BDD library for Scala

Im using the Specs BDD library for writing Scala unit tests (http://code.google.com/p/specs) .In my code if i want to assert that a throws an exception of type ClassNotFoundException, then i can write the following code:

a must throwA[ClassNotFoundException]

However,i want to test the reverse case,i.e.i want to assert that a "does not" throw an exception of type ClassNotFoundException.

I tried using not negation matcher, as follows:

 a must throwA[ClassNotFoundException].not

But that didnt work. Im getting compilation errors. So, is there any way i can assert that an exception of type ClassNotFoundException for example, is not thrown ?

Please Help Thank You