and

We are grateful for the discussions with Roel Bloo, Tijn Borghuis, Erik Poll and Phil Wadler and for the helpful remarks received from them. In particular, we are grateful to Phil Wadler who has recommended that we dispose of 's and 's as in (x)( y)y and write (x)[y]y instead. We are also grateful to Peter Peters for his help concerning Latex and e-mail which enabled us to keep exchanging drafts and corrections of the present paper. Last but not least, we are grateful to the anonymous referees for their useful comments.