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, ANR (06PCVI001302) and by a Microsoft PhD fellowship grant to P.A.P. We thank J. Whitehead for her suggestions and reading of the manuscript. P.A.P. realized the simulations, mechanical cuts, and imaging; P.A., the indent experiments and imaging; and A.C.B., the ShiSna experiments, This work was funded by the Human Frontier Science Program (RGP0014/2006) and

, , 2009.

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