Unfortunately the incorrect version of figure 3 was published in this paper, please see the correct version below.
![](https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary-alt:20160713224149-76848-mediumThumb-S0021859610001139_fig1g.jpg?pub-status=live)
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Published online by Cambridge University Press: 13 December 2010
Unfortunately the incorrect version of figure 3 was published in this paper, please see the correct version below.