A.37 library(ugraphs): Unweighted Graphs
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Availability::- use_module(library(ugraphs)).(can be autoloaded)
Sourcereachable(+Vertex, +Graph, -Vertices)
Unify Vertices with the set of all vertices in Graph that are reachable from Vertex. Example: 
?- reachable(1,[1-[3,5],2-[4],3-[],4-[5],5-[]],V).
V = [1, 3, 5]

ClioPatria (version V3.1.1-51-ga0b30a5)