OK, I see that `Svg = Markup = MarkupM ()` is indeed not an instance of `Show`, so we can no longer derive a `Show` instance.  We could implement a custom `Show` instance which simply serializes the SVG.  Probably that's what we should do for now, to avoid breaking `diagrams-builder` at the moment.  But this is not really in the spirit of `Show`.  Probably what we should actually do is switch to using `Hashable` instead of `Show` in `diagrams-builder`.