list circularity

[prieur@smart.irit.fr (Pascal PRIEUR)]

Dear Sir,

I would like to know if someone has already work on an implementation
of a functional language in Centaur (ML,Lisp).
I have a problem to implement the letrec operator with Typol.
I dont know how to express the circularity of a list, using something 
like the lisp 'rplac'.

example:
(letrec 
(x == [1 y])
(y == [2 x])
then x) 
should give me the result [1 [2 [1 [2 ..... ]]]]

Please give us a help!

	Thanks in advance.