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For instance, the directed graph below is a DAG even though there is an (undirected) cycle B-D-E-C-B because any traversal of the cycle requires that one travels against the direction of at least one edge.
Stemmata for transmissions that are not contaminated are DAGs (as long as they are oriented) can be represented as DAGs, and each case of contamination will result in a cycle in the graph.
Illustration
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