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ELVIS contains 48 dark-matter halos of masses similar to the MW or M31 ($\mvir = 1.0 - 2.8 \times 10 ^ {12} \msun$), with a median virial radius of $\rvir \approx 300 \kpc$.   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.  We use all ELVIS  halos, given the lack of strong difference in the satellite  virial-infall timesfor satellites  in the paired versus isolated 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), subhalo,  we assign a virial mass, $\mvir$, and radius, $\rvir$, using the evolution of the virial relation from \citet{BryanNorman1998} for our $\Lambda$CDM cosmology. \citet{BryanNorman1998}.  We define a ``subhalo'' as a halo whose center is inside $\rvir$ of a host halo.  When a (sub)halo passes within $\rvir$ of a host halo, the (sub)halo becomes its ``satellite'' and experiences ``virial infall''.  For each (sub)halo, we compute the peak mass, $\mpeak$, that it reached along the history of its primary progenitor.  To assign stellar mass, In order to match subhalos to observed satellites,  we use assign $\mstar$ to subhalos using  the relation from abundance matchingto ELVIS subhalos  in \citet{GarrisonKimmel2014}, which reproduces the observed mass function at $\mstar < 10 ^ 9 \msun$ in the LG if one accounts for observational incompleteness \citep{Tollerud2008, Hargis2014}. Admittedly, While  the relation between stellar mass $\mstar$  and subhalo mass $\mpeak$  for dwarf galaxies is remains  highly uncertain, likely with significant scatter.  However, as scatter, in this work the relation is important \emph{only} in assigning virial-infall time distributions to satellites in a 1-dex bin of $\mstar$.  As  \citet{Wetzel2015} showed... showed, satellite infall times generally change by $<10-20\%$ over $\sim 1$ dex in $\mstar$.  See \citet{GarrisonKimmel2014} for more details on ELVIS, and \citet{Wetzel2015} for more details on computing satellite infall times.