I put an experimental SLICE in the inbox: let a Rectangle merge in
place (I called this swallow:)
This has two advantages:
- avoid repeated Object creation when we just want the gross result
- avoid closures writing to outer temps

IMHO, generalizing this kind of policy could have a measurable impact
on GUI speed.
However, this is against current policy to never change a Point nor
rectangle in place, so I let gurus judge if worth or not.

cheers

Nicolas