> That's just broken, and will lead to horrible bugs.

You mean you think people will use this? Thanks :)

Here's how I was thinking of fixing it:

(setf nol 'nol) ; make nol evaluate to itself

(defun my-null (x) ;redefine null (if (or (null x) (eql nol x)) t null))

Then, change add so that if it's adding a nil, it changes the sentinel at the end of the linked list part of the list to nol.

So (trellis 1 2) returns (1 . ((2 . NIL) . NIL))

And (add nil (trellis 1 2)) returns (1 . ((2 . NIL) . NOL))

As soon as someone adds something other than a nil to a cdr of a cons in a tree, add changes nol back to nil.