Note that the *'s denote various mathematical symbols that use letters in both superscript and subscript which do not copy correctly, the fractions have been written in a single line and the subscript letter 'k' and superscript letter 't' and 'k' in the abstract have not copied correctly.
RGMIA digitization. CAVAL 2011Abstract removed  Lyn 26/9/12. Permission granted, in perpetuity, RGMIA version. liz 7/9/12