The publisher apologises that upon publication of this article a typographical error was introduced with [-4pt] being added to equation 2.1.
The equation should read:
![](https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20211119101346430-0866:S0022112021009009:S0022112021009009_eqn1.png?pub-status=live)
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Published online by Cambridge University Press: 19 November 2021
The publisher apologises that upon publication of this article a typographical error was introduced with [-4pt] being added to equation 2.1.
The equation should read: