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ELVIS contains 48 dark-matter halos of masses similar to the MW or M31 ($\mvir = 1.0 - 2.8 \times 10 ^ {12} \msun$).  Half of these halos are located in zoom-in regions that were selected to contain a pair of halos that resemble the masses, distance, and relative velocity of the MW-M31 pair, while the other half are single isolated halos matched in masses to the paired ones.  Here, we We  use all halos, finding no significant differences given the lack of strong difference  in satellite infall the virial-infall  times for satellites  in using just the  paired or versus  isolated halos, in agreement with \citet{Wetzel2015}. halos \citep{Wetzel2015}.  ELVIS identifies dark-matter (sub)halos using the six-dimensional halo finder \textsc{rockstar} \citep{Behroozi2013a} and constructs merger trees using the \textsc{consistent-trees} algorithm \citep{Behroozi2013b}.  For each halo that is not a subhalo (see below), we assign a virial mass, $\mvir$, and radius, $\rvir$, using the evolution of the virial relation from \citet{BryanNorman1998} for our $\Lambda$CDM cosmology.