Without it, the structure allowed the user to create a graph like this: [x] Task ├──[x] Sub-task (1) │ ├──[x] Sub-sub-task (2) │ └──[x]

Without it, the structure allowed the user to create a graph like this: [x] Task ├──[x] Sub-task (1) │ ├──[x] Sub-sub-task (2) │ └──[x] Sub-sub-task ├──[x] Sub-task (3) │ ├──[x] Sub-sub-task (2) │ └──[x] Sub-sub-task └──[x] Sub-task by creating a link from (3) to (2). The tree command would actually omit the second occurrence of (2), since the algorithm visited each node just once.