A.60 library(ugraphs): Graph manipulation library
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Availability::- use_module(library(ugraphs)).(can be autoloaded)
Sourcereachable(+Vertex, +UGraph, -Vertices)
True when Vertices is an ordered set of vertices reachable in UGraph, including Vertex. Example: 
?- reachable(1,[1-[3,5],2-[4],3-[],4-[5],5-[]],V).
V = [1, 3, 5]

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