I can't set gud-key-prefix. How set this variable??

[Christian Seberino]

I tried to set gud-key-prefix and was not able to do it.  Is there
some special way to set this variable that is unlike other variables???

Here is one attempt to set variable:

 (setq-default        gud-key-prefix           "C-x" )

Here is the error message when I start gdb:

 Key sequence C - x C-l uses invalid prefix characters

This is odd since I'm not even altering gud-key-prefix
from its original value!?!?

Chris