faint willow
This would be an expansion of the prefabs tool.
It takes the prefab selection box, and sees what can be mirrored on the other side of the mirror line.
Sub-options here are to remove and replace all obstructing parts, or to append, which only adds parts where no obstruction exists, and ideally attempts shorter versions of the same obstructed parts, if applicable.
I haven't had any major moments concerning this - but considering the scale that people build in FTD, I am guessing that there have been horror stories.
This should greatly reduce pain from forgotten mirror moments, and it could also have use for merging two 'forms' together.
mossy wigeon