[agumonkey]

Very interesting analogy. Many 3D modeling programs had undos based on clean abstractions (history of transformation matrices for object P/O/S, stack of operators to influence geometry) but it often broke when you go below. Houdini (one that was fairly dogmatic about being 'procedural') had to reify vertices/edges modification as an Edit node in its object graph to stay coherent without losing information (or making it a second class citizen).