Definition
For a directed graph D = (V, A), the reachability relation of D is the transitive closure of its arc set A, which is to say the set of all ordered pairs (s, t) of vertices in V for which there exist vertices v0 = s, v1, …, vd = t such that (vi - 1, vi ) is in A for all 1 ≤ i ≤ d.
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