Circular node references #42
Description
Handle circular node references where a node is also the parent of its own parent. Here's an example where nodes 2 and 3 are circular:
| Node | Parent |
|---|---|
| 1 | NA |
| 2 | 3 |
| 3 | 2 |
| 4 | 1 |
| 5 | 2 |
This is an invalid tree structure, so this should raise an error and stop execution.