Given a directed graph $G=(V,E)$ and a source node $s \in G$, an incremental single-source reachability algorithm maintains the set of nodes reachable from $s$ (i.e., all nodes $v$ for which there is a path from $s$ to $v$ in the current version of $G$) during a sequence of edge insertions, with sensitivity 1, i.e. when 1 edge is inserted.