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Predicate vertices_edges_to_ugraph/3
 vertices_edges_to_ugraph(+Vertices:list, +Edges:pairs, -UGraph) is det
Create a UGraph from Vertices and Edges. UGraph must unify with the corresponding S-representation. Note that vertices that do not appear in any of the Edges appear in UGraph as Vertice-[]. The set of vertices in UGraph is the union of Vertices and all vertices that appear in the Edges pairs. 
?- vertices_edges_to_ugraph([],[1-3,2-4,4-5,1-5], L).
L = [1-[3,5], 2-[4], 3-[], 4-[5], 5-[]]

In this case all vertices are defined implicitly. The next example shows three unconnected vertices:

?- vertices_edges_to_ugraph([1,2,6,7,8],[1-3,2-4,4-5,1-5], L).
L = [1-[3,5], 2-[4], 3-[], 4-[5], 5-[], 6-[], 7-[], 8-[]]
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