Some time ago I've made relatively simple SVG converter that handled very small subset of this format (paths, color & opacity). What this piece of software was doing is "compressing" of misc optimizations: G-elems got fused into PATH etc. The problem is that it probably won't work if I were to make it more conformant: there's too much cross-references that may depend on those fused elements (USE may point to G, which will be compressed; values of some atts may point to other elements atts). So, maybe someone can share a high-level view on how this have been done in Inkscape? At first I thought that simple parsing-and-converting would be enough, but it seems that I'll have to make a full in-memory copy of the document...