The publisher apologises for the error on page 6 of this article where an incorrect expression of covariances was given in the last line of Section 2 (the ninth line from the top of the page). The term should read:
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Published online by Cambridge University Press: 03 October 2016
The publisher apologises for the error on page 6 of this article where an incorrect expression of covariances was given in the last line of Section 2 (the ninth line from the top of the page). The term should read: