The joke would be a little better if you didn't explicitly as it was a lisp. Gotta leave a moment of epiphany for the listener.
> A Chinese spy manages to steal the last 50MB of the program governing U.S. missile launches. Fortunately, it was all closing parentheses.
or
> A Chinese spy manages to steal the last 50MB of the program governing U.S. missile launches. Fortunately, it was Lisp.
A Chinese spy stole the last 48MB of the Abram's targeting system. To our good fortunes this was only the closing parentheses.
Fortunately it was in Lisp, so it was all closing parentheses.