how to subtract coordinates between 2point lists

Hello GH world!
i can not finde peace...GH´s algebra is different than mine!
here an extract of my definition-
the task is mouldy easy: check if the z-coordinates of the points on the line are bigger than the ones of their corrispective projection on the arch.

What do I do wrong?
-the algorithm of the function is calculated in the shortest list modality.
-the coordinates to subtract one to each other belong to the same path.

I beg your pardon if I am insulting your mind...but I cannot find help on the Primer..
tnx a lot,adele