Here is my code for the planet simulation. The struct xyvect, declared at
the top of the file, uses arithmetic operator overloading.
It could be placed in a separate file if required.

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George Saklatvala
Room 821/823 Rutherford Building,
Cavendish Laboratory,
J. J. Thomson Avenue,
Cambridge CB3 0HE.
Telephone: (+44) (0)1223 766468
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David MacKay enhanced this example by rewriting the angular momentum
function J().