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This paper presents an original structured integration scheme for Lagrangian systems undergoing elastic impacts due to unilateral constraints. In deriving the method, a backwards error analysis approach establishes a discrete time conservation law for impacts using the notion of a modified system Hamiltonian. Shortcomings of existing simulation methods that do not use this conservation law are demonstrated both analytically and in simulation. Relative to these methods, our scheme provides advantages in terms of discrete time energy behavior, accuracy of trajectories, and consistent order of accuracy of the method.