```python tables = tskit.TableCollection(1) for _ in range(10): tables.nodes.add_row(flags=tskit.NODE_IS_SAMPLE, time=0) ts = tables.tree_sequence() print(ts) tree = tskit.Tree(ts) tree.first() print(tree.roots) # Gives [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] tree = tskit.Tree(ts, root_threshold=2) tree.first() print(tree.roots) # Gives [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] ``` The list of roots should be empty in the second case.