A.42 library(ugraphs): Unweighted Graphs
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Availability::- use_module(library(ugraphs)).(can be autoloaded)
Sourcetranspose_ugraph(+Graph, -NewGraph)
Unify NewGraph with a new graph obtained from Graph by replacing all edges of the form V1-V2 by edges of the form V2-V1. The cost is O(|V|^2). Notice that an undirected graph is its own transpose. Example: 
?- transpose_ugraph([1-[3,5],2-[4],3-[],4-[5],
              5-[],6-[],7-[],8-[]], NL).
NL = [1-[],2-[],3-[1],4-[2],5-[1,4],6-[],7-[],8-[]]