DAG in which the set of vertices reachable from any vertex induces a tree, or a poset that does not have four items a, b, c, and d forming a diamond suborder with a ≤ b ≤ d and a ≤ c ≤ d but with b and c incomparable to each other
DAG in which the set of vertices reachable from any vertex induces a tree, or a poset that does not have four items a, b, c, and d forming a diamond suborder with a ≤ b ≤ d and a ≤ c ≤ d but with b and c incomparable to each other (en)