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The CP -violating asymmetry assl is studied using semileptonic decays of B0s and B¯¯¯0s mesons produced in pp collisions at a centre-of-mass energy of 7 TeV at the LHC, exploiting a data sample corresponding to an integrated luminosity of 1.0 fb−1. The reconstructed final states are D±sμ∓, with the D±s particle decaying in the ϕπ± mode. The D±sμ∓ yields are summed over B¯¯¯0s and B0s initial states, and integrated with respect to decay time. Data-driven methods are used to measure efficiency ratios. We obtain assl=(−0.06±0.50±0.36)%, where the first uncertainty is statistical and the second systematic.