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Three-dimensional cross power spectrum of the 21cm emission measured at the Green Bank Telescope and the optical density field traced by the WiggleZ redshift survey. Negative points are shown with reversed sign and a thin line. The solid line is the mean of simulations based on the empirical non-linear model of Blake et al. (2011) processed by the same pipeline. The overall amplitude is consistent with a measurement of Omega_HI~4*10^(-4) at z=0.8, similar to the value measured at z=0.