Tcl has become a lot harder to debug since NRE's adoption.
The problem is intrinsic to the nature and goals of the NRE: C keeps a who called me stack, NRE does its best to replace it with a who do we call next stack. Most debugging tools like gdb are designed for C
The student would research different possibilities for assisting the developer to debug Tcl's core, including a simpler inspection of core dumps, and implement one of them.
One idea might be to devise a tracing mode, enabled with special compile flags, that would maintain a who called me stack (in memory? on file?). A second step would then be to arrange for the tracing output to be inspected from gdb.