improve gud-gdb completion

[Stephen Leake]

I'd like to improve completion in gud-gdb.

Currently, it completes on names in the buffer being debugged, but it
doesn't include "." in the name, so if I'm trying to print
"Incremental_Parser.Parse_Errors.Length", it only completes to
"Incremental_Parser".

However, I can't figure out exactly what elisp code is harvesting names
from the buffer.

TAB is bound to completion-at-point.

completion-at-point-functions is
(gud-gdb-completion-at-point comint-completion-at-point t)

(default-value 'completion-at-point-functions) is
(tags-completion-at-point-function)

None of those completion functions look at the text in buffers.

What am I missing?

-- 
-- Stephe