- Correspondence to Dr Vincent A Knight, School of Mathematics, Cardiff University, Senghennydd Road, Cardiff CF24 4AG, UK;
- Accepted December 30, 2011
- First published February 14, 2012.
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.