J. Ryckebusch, V. Van der Sluys, M. Waroquier, L.J.H.M. Kester, W.H.A. Hesselink, E. Jans, A. Zondervan Physics Letters B 333 (3-4), 310-3151994A1Published while none of the authors were employed at the CMMDOI http://dx.doi.org/10.1016/0370-2693(94)90147-3