The parents of a vertex v in a digraph D are all the vertexSet u in D such that u,v is an edge of D. In other words, the parents of v are all the vertexSet that have edges coming out of them that point at v.
i1 : D = digraph({a,b,c,d,e},{{a,b},{b,c},{b,d},{e,b}}); |
i2 : parents (D, b) o2 = set {a, e} o2 : Set |