A.56 library(ugraphs): Graph manipulation library
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Availability::- use_module(library(ugraphs)).(can be autoloaded)
Sourceugraph_union(+Graph1, +Graph2, -NewGraph)
NewGraph is the union of Graph1 and Graph2. Example: 
?- ugraph_union([1-[2],2-[3]],[2-[4],3-[1,2,4]],L).
L = [1-[2], 2-[3,4], 3-[1,2,4]]