Thulani Earnshaw,
http://link.springer.com/chapter/10.1007%2F978-3-319-01282-7_4#close link.springer.com
http://link.springer.com/chapter/10.1007%2F978-3-319-01282-7_4#close link.springer.com
http://link.springer.com/chapter/10.1007/978-3-319-01282-7_7
http://link.springer.com/chapter/10.1007/978-3-319-01282-7_7
http://link.springer.com/chapter/10.1007/978-3-319-01282-7_7