copying userdata objects?

["J. Perkins" <jason@...>]

I have added a couple of 3D object types (vector, matrix, 
quaternion) to Lua as userdata objects. The default behavior of a 
statement like:

	vector2 = vector1

is to set both values to reference the same userdata object. But I 
would like my math objects to behave like the simple data types, in 
that assignment makes a *copy* of the object, and changing vector2 
would have no effect on vector1. Is there an easy way to accomplish 
this in Lua? There doesn't seem to be an assignment tag method. The 
only way I can think to make it work is to create a new userdata 
object on every call to "settable", but that would generate a lot of 
garbage. If I could detect the assignment, I could at least use a 
refcount scheme to only duplicate the necessary objects.

Thanks,
Jason