Résumé : After a brief introduction to the Circus programming language, we present a simple type set to model XML structures. We then describe a transformation that takes a mail as input and produces a reply, showing how subtyping is used in order to refine the type control and specialize the transformation. Conclusions are drawn both on our (easy to use but clearly limited) XML data model and on Circus itself ; expected qualities of the language are verified ; the need for some new features is expressed. Finally, we sketch some language extensions, a richer model for XML structures, and explain our choices and expectations.