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3. Materials and methods are available on Science Online.
4. We thank N. Barré S. Blancher R. Joyce J. Spaggiari J. Troscianko and J. Watts. Supported by the Biotechnology and Biological Sciences Research Council (BB/C517392/1) the Province Sud the Rhodes Trust (C.R. and L.A.B.) and Linacre College (C.R.). Full acknowledgments are provided in ( 3 ).