The math behind the Trim command is somehow able to distinguish “inside” from “outside” , “left” from “right” or “positive” from “negative”.
On a simple case like this:
during the first click the surface is already split into all possible fragments… the next clicks are needed only to know where to remove the fragments.
If we were able to invert that part, and click where to keep the fragment, we could do something like this: (imagine a ctrl key being hold-pressed during the click… or something)
This is a simple 2D case but the cases where this functionality would be awesome is during 3D cuts.
Very often, during 3D cuts with many surfaces/polysurfaces/curves, i have to click on all the fragments but one to achieve my shape…
Edit: as it would work in “negative” it should have some sort of visualization aid.
Trim: everything stays unless clicked
Negative trim: everything goes unless clicked
The first fragment clicked make all the others disappear(?), it should be possible to click other fragments after anyway.