Well, if you don't need to make changes after mirroring, then you can do this:
1) install (if not installed) the "copy as new furniture" plugin
2) select everything in your design, then use the "copy as new furniture" option
3) open a new document and paste the copied desing, which will appear as a single object
4) edit the object and mark "mirrored shape" checkbox
That's all.
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If a long walk every day were healthy, mailmen would be immortal.
