I have an object I'm interested in, call it A.  It's in a container C.
It is displayed on the screen with a morph M1, which is embedded in other 
morphs.

I have another object B, who display includes the morph M2.
If someone drags M1 to M2 and drops, I want to put A into one of B's variables.

What's the best way to do this?  It looks as if the drag n drop protocol is 
designed for removing M1 from its morphs and placing it somewhere else.  I 
want to leave it where it is.

All I really need is to copy the object reference.
I could also copy M1 and replace M2 with M1.  This might even be preferable.

I see various stuff about "parts bins" that might be relevant (this is a 
lot like pulling items off the flaps), but exactly how that works is 
unclear (e.g. parts bin vs donor).  I don't see a place to override what 
gets dragged (so I could return a copy).

I'd appreciate any pointers.