Validate data entry in a Simple List?

[Kenneth Swanson <kswanson watson wustl edu>]

Hey all,

I'm writing a small application that will allow people to manipulate data in a list.  Right now I'm using a Gtk2::Ex::Simple::List for it, which 
is working well, and I set up the cell renderers to be editable, and it all works fine.

My problem now is that I want to add some validation logic to some columns, where if a user tries to type in an invalid value it will pop up an 
error and refuse to store the value in the list.  I can detect the new value correctly by connecting a callback to the CellRendererText's edited 
signal, but I can't seem to stop the SimpleList from storing the new value.

Is there a way to stop the new value from getting stored, short of just over-writing it with the old value?  Or is this something too complex 
for a 'simple' list?

Thanks for your help,
-Ken