/usr/local/lib/swipl/library/ugraphs.pl
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 add_edges(+Graph, +Edges, -NewGraph)
Unify NewGraph with a new graph obtained by adding the list of Edges to Graph. Example: 
?- add_edges([1-[3,5],2-[4],3-[],4-[5],
              5-[],6-[],7-[],8-[]],
             [1-6,2-3,3-2,5-7,3-2,4-5],
             NL).
NL = [1-[3,5,6], 2-[3,4], 3-[2], 4-[5],
      5-[7], 6-[], 7-[], 8-[]]