I guess I misunderstood your intention. I thought you wanted to be able to redefine the existing toplevel used by the arc interpreter itself, not define a completely new toplevel to be used in user code... (and yes, the latter is quite trivial).
> I thought you wanted to be able to redefine the existing toplevel used by the arc interpreter itself
As opposed to emulating the existing toplevel in user code?
Allowing a redefinition of 'eval to work on the REPL will probably be OK, but still, there would be an impedance mismatch with functions that were defined before 'eval was redefined.
