I would like to report/say that "s 1" and "s 2" are incompatible, here's how.:
Place s 2 (first point (node) is low, then second is middle) and then continue with s 1 (last point is high).
when you these 2 segments in a row and in a slope, you can see that it'sn't slope, but at the point where road changes there is flat road.
It's hard to describe, and I can't post picture here, (unlike in reddit).

Yea and, can somebody please explain why some roads'ren't compatible? (example "T 2 3 L" and s 1 + s 2)