A.56 library(ugraphs): Graph manipulation library
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Availability::- use_module(library(ugraphs)).(can be autoloaded)
Sourceedges(+Graph, -Edges)
Unify Edges with all edges appearing in Graph. Example: 
?- edges([1-[3,5],2-[4],3-[],4-[5],5-[]], L).
L = [1-3, 1-5, 2-4, 4-5]