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}});
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i2 : parents (D, b)
o2 = set {a, e}
o2 : Set
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The object parents is a method function.