Unbinding root definitions which hide definitions of a module

[Greg Badros <gjb at cs dot washington dot edu>]

Suppose I have a:

(define foo 1)

in a module.

and I later do a:

(define foo 2)

in the-root-module.

Is there any way to undo the definition in the-root-module to permit the 
definition of foo in the other module to be visible?  undefine does not
seem to do this, and I'm not sure if I'm wrong to expect that it would.
(It could just be that I'm working too late! :-) ).

Thanks,
Greg