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We report measurements of partial branching fractions for inclusive charmless semileptonic B decays Bbar --> Xu l nubar,
and the determination of the CKM matrix element |V_{ub}|. The analysis is based on a sample of 467 million Upsilon(4S) --> BBar
decays recorded with the BaBar detector at the PEP-II e^+ e^- storage rings. We select events in which the decay of one of
the B mesons is fully reconstructed and an electron or a muon signals the semileptonic decay of the other B meson. We measure
partial branching fractions DeltaB in several restricted regions of phase space and determine the CKM element |V_{ub}| based
on four different QCD predictions. For decays with a charged lepton momentum p_l^* > 1.0 GeV in the B meson rest frame, we
obtain DeltaB = (1.80 \pm 0.13 (stat.) \pm 0.15 (sys.) \pm 0.02 (theo.)) \times 10^{-3} from a fit to the two-dimensional
mX-q^2 distribution. Here, mX refers to the invariant mass of the final state hadron X and q^2 is the invariant mass squared
of the charged lepton and neutrino. From this measurement we extract |V_{ub}| = (4.33\pm 0.24 (exp.) \pm 0.15 (theo.)) \times 10^{-3}
as the arithmetic average of four results obtained from four different QCD predictions of the partial rate. We separately
determine partial branching fractions for B^0 and B^- decays and derive a limit on the isospin breaking in Bbar --> Xu l nubar decays.
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