> Most CAD programs I've used store a tree of features/actions used to get 
> to the final shape, this means you can go back and edit any of those 
> previous steps.

That sounds like a more logical way to work.

Presumably CAD also allows you to do things like "find a point 3/5 along 
the length of this edge". SketchUp automatically marks the 1/2 point for 
you, but for any other point you've got to get out a calculator.

Other annoying SketchUp features include the lack of the ability to 
manipulate a circle be its center. (Because, of course, once you've 
created it, it's not a circle any more. It's 24 unrelated edges.) And 
the best feature of all is the way surfaces "stick" to each other as 
soon as they touch...

> I saw partly this kind of behaviour in 3D Studio Max 
> (or whatever it's called now), in that you could keep two separate 
> objects and see the intersection of them, but most mesh editors just 
> store the mesh and no extra data.

Yeah. How SketchUp does intersection is that you line up the two 
objects, press "intersect", and then delete the geometry you don't want. 
Manually. Which, for curved surfaces, takes a really long time. 
(Because, naturally, those surfaces aren't really curved. Stupid 
idea...) Also, you can't delete any item smaller than a certain size on 
screen. You have to zoom in first. And the minimum size is significantly 
larger than you'd expect.

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