In terms of graph isomorphism complexity classes Trees have a polynomial time algorithm and Directed Acyclic Graphs (DAG's) do not (so far).
What about Poly-trees (oriented trees)? These are DAG's where the possible paths from a node are all trees. Unlike tree nodes, Poly-tree nodes can have several parents.
Are polytrees in the isomorphism complexity class of DAG's or Trees, so far?
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