 Some authors restrict the phrase "directed tree" to the case where the edges are all directed towards a particular vertex, or all directed away from a particular vertex (see arborescence).A "polyforest" (or "directed forest" or "oriented forest") is a directed acyclic graph whose underlying undirected graph is a forest.In other words, if we replace its directed edges with undirected edges, we obtain an undirected graph that is acyclic.