r = isSink (D, v)
A vertex v of a Digraph D is a sink if v has no children
i1 : D = digraph({a,b,c,d,e},{{a,b},{b,c},{b,d},{e,b}});
i2 : isSink (D,b) o2 = false
i3 : isSink (D,d) o3 = true
The object isSink is a method function.