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-[]]
