Define a graph timeline to be a sequence of graphs $G_1$, $G_2$, $\dots$, $G_t$ on a common set of vertices $V$ of size $n$ such that the graph $G_i$ is obtained from $G_{i-1}$ by adding or deleting a single edge. Given a graph timeline, preprocess the timeline to build a data structure that may answer "exists" connectivity queries of the form: are vertices $u$ and $v$ connected by a path in any of $G_a$, $G_{a+1}$ ,$\dots$ ,$G_b$