This document has benefited from contributions made by participants during the electronic conference on the subject. Their contributions are highly acknowledged. Special thanks are extended to R.N. Roy and R.V. Misra for their contribution to the conceptualization, organization of the electronic conference and initiation of this review. Thanks are due to J. Poulisse for his constructive suggestions. Thanks are also due to E.M.A. Smaling and J.P. Lesschen for their suggestions and contributions.
