The topology identification routine doesn't differentiate between upper and lower single null: https://github.com/boutproject/xBOUT/blob/cb87f2319e1f2dbe2442fe76ff141a3b0b39b5df/xbout/region.py#L309 It's important to differentiate between them because LSN has the inner target at the ylow side, while USN has the inner target at the yup side.
The topology identification routine doesn't differentiate between upper and lower single null:
xBOUT/xbout/region.py
Line 309 in cb87f23
It's important to differentiate between them because LSN has the inner target at the ylow side, while USN has the inner target at the yup side.