1 A polytree (Rebane and Pearl, 1987) is a multitree whose underlying graph is a tree.
the out property  An ordered digraph is weakly projective (PROJ W ) if for all vertices i, j and k, if k → j → i, then either {i, j} < k or {i, j} > k. the constraint, aka the outside-to-inside constraint (Yli-Jyrä, 2005), states that no outgoing arc of a vertex properly covers an incoming arc.