question about pdb assignment statements

[Steve Ferg]

In this little script:

 <pre>
 import pdb
 pdb.set_trace()
 def main():
     xm = 123
     print("Hello,world!")
 main()
 </pre>

When I run this, I use pdb to step through it until I reach the point
in main() where the xm variable has been initialized, and then I try
to use pdb to reset the value of xm, and I can't.

Does anybody know why?

As I understand the documentation,  http://docs.python.org/library/pdb.html
I *should* be able to do this.

 [!]statement
 Execute the (one-line) statement in the context of the current stack
frame.

Is there something about "in the context of the current stack frame"
that I don't understand?  Or is it a bug (or a limitation) in pdb?