A.56 library(ugraphs): Graph manipulation library
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Availability::- use_module(library(ugraphs)).(can be autoloaded)
Source[semidet]top_sort(+Graph, -Sorted)
[semidet]top_sort(+Graph, -Sorted, ?Tail)
Sorted is a topological sorted list of nodes in Graph. 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]

The predicate top_sort/3 is a difference list version of top_sort/2.