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tokens: The author wishes to thank Derek Ward-Thompson for valuable discussions, the anonymous referee for a detailed and constructive review which greatly improved the content of this paper, and Chris Powles for proof-reading the derivations. The author wishes to thank the Science and Technology Facilities Council for studentship support under grant number ST/K501943/1 and post-doctoral support under grant number ST/K002023/1 while this research was carried out. This research has made use of the NASA Astrophysics Data System.