Filtering boundary surfaces - floor

Hi guys,
i m trying to find out how can i filter surfaces on floor - but I want to filter only vertical surfaces (i find on clockwork Surface.FilterByOrientation - this is okay) and i want to filter only “boundary” surfaces of floor, but i dont know how.
I have very simple dateset with one floor and somewhere inside this floor is opening (or more openings).
I found this node - Springs.Collector.ElementSketch and i can see boundary lines here, but I don’t know how to set “condition” to filter by bool mask, if i want to know whitch line(curve) from list is located somewhere on the edge of the surface

test_01.dyn (11.7 KB)

so much thanks for your help

Some pseudo code to help:

Surface.NormalAtParameter with a parameter value of 0.5, 0.5 to get the normal for each face
Vector.Z to get the Z component of the vector.
== node to test if the node equals 0.
List.FilterByBoolMask with your original surfaces as the list and the results of the == node as the mask.
The vertical surfaces should be in the ‘IN’ output.