A.42 library(ugraphs): Unweighted Graphs
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Availability::- use_module(library(ugraphs)).(can be autoloaded)
Sourcetop_sort(+Graph, -Sort)
Generate the set of nodes Sort as a topological sorting of Graph, if one is possible. A toplogical sort is possible if the graph is connected and acyclic. In the example we show how topological sorting works for a linear graph: 
?- top_sort([1-[2], 2-[3], 3-[]], L).
L = [1, 2, 3]