How's this:

Take the times $t_{pre}$ to be of the form $t_{pre} = T_{0} + iT$, where $i$ ranges over
some set of integers of the form $\{0, 1, 2, . . . , N\}$, and we allow the possibility
that $N$ is infinite and place no restriction on the sign of $T$, the interval size, though
we take $T \ne\ 0$; since the case $T = 0$ boils down to there being only one value of $t_{pre}$, $T_{0}$, in which case $V(t)$ trivially becomes $(\epsilon_{0}/\tau)e^{(t - T_{0})/\tau}$. (I'm dropping your subscripts to $\tau$ for convenience, i.e. to make typing
slightly easier. Then in general we have %V(t) = \sum_{i = 0}^{N}(\epsilon_{0}/\tau)e^{(t-T_{0} - iT)/\tau}$