> * this line needs to be 1/3 the length of that line
> * these set of lines should start 50mm left of this line
Remember that you're working with points. So, to express the length of a line, you need something like (x1-x2)^2 + (y1-y2)^2 .
If you just wanted to do things like "this point is +(0,10) of this other one", you're doing a really simple constructive system, which is easy to evaluate, but not anywhere near as flexible or powerful. ImplicitCAD/OpenSCAD trivially do that, just without a nice interface.
My my view is probably way too simplistic, but I'm really wondering how you would end up with quadratic terms if you need to define stuff like:
* this line needs to be 1/3 the length of that line
* these set of lines should start 50mm left of this line
I can see how trigonometric expressions could be useful to specify angles (although a constant there covers 99% of the user-cases?).