I suppose technically it's a forest, because there can be multiple root tasks. If you want to simulate a directed acyclic graph of dependencies (examples aplenty), you have to find a supporting tree (would that always be a subgraph?) that is represented directly in TBB (which doesn't do dags), and fill in the missing dependencies by adding empty_task children of the dependent tasks to be explicitly spawned by their dependencies on completion (or use release_ref() as implemented in the "Pipeline" thread).
