In this paper we inadvertently allowed a typographical error in the formula for calculating the metaprism twist angle to slip through the net. However, the correct formula was used for the calculations, and so all published values are correct. The error is on page 441 and involved defining the Y2 term using the O(1)y atom coordinate instead of that for O2(y). Thus the correct formula is: graphic

We thank Taylor Sparks of the School of Engineering and Applied Sciences, University of Harvard, USA for pointing out this error to us and apologise to all readers for this careless mistake.

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