Trim a surface in GH?

I am struggling to get silly flat surfaces to fillet properly.
To increase my chances I want to make the common edge an untrimmed edge for both surfaces.
Now I run into the issue of trying to do a “Boundary Surface”, but using an underlying surface of my own, and not whatever silly surface Rhino will generate.
OR try to trim my surface with the curves, but I realize there appears to be no way to do this in GH !!!

Never mind. I just discovered the “Retrim” component