Neat TYPEP Trick
How do you test if an object is a cons that has the desired symbol in the car?
(typep x '(cons (eql :foo)))
(and (consp x) (eq :foo (car x)))
is essentially just as short...
I still find cons types neat, even if they're a nightmare when it comes to type derivation, but that's a different matter. Some nightmares aren't all bad.