R help - Jan 2013 - modify functions in imported namespace in a new package

Hello all,
I am developing an R package. My package uses several
classes and functions from another package (package A). Normally, I could just
specify package A as imports or depends in my DESCPRITION and NAMESPACE files.
One function I use is F, which calls a bunch of other
functions including G in package A. To use function F in my package, I have to
modify function G first. Therefore, I rewrite function G first and do
assignInNamespace("G",
G, envir="A") in the source R file, either with or without .onLoad
function. Either way, the package was checked and installed free of error. But
after loading, my package still calls the original function G (through F) in
package A, instead of the version I supplied. 
Note that I may do assignInNamespace after my package gets
loaded in a user session, it works perfectly fine. I realize that I may
manually include in my package all the sources for relevant classes and
functions from package A and rewrite function G only. But that?s too awkward
and does not gives credit to package A.
I would really appreciate any thoughts and suggestions on
why assignInNamespace didn?t work in my source package, and how to solve it.
Thanks!
Heyi

Any thoughts or suggestions? This does not seem be a trivial
issue. Thanks!

----- Original Message -----
From: heyi xiao <xiaoheyiyh at yahoo.com>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc: 
Sent: Wednesday, January 2, 2013 3:22 PM
Subject: modify functions in imported namespace in a new package

Hello all,
I am developing an R package. My package uses several
classes and functions from another package (package A). Normally, I could just
specify package A as imports or depends in my DESCPRITION and NAMESPACE files.
One function I use is F, which calls a bunch of other
functions including G in package A. To use function F in my package, I have to
modify function G first. Therefore, I rewrite function G first and do
assignInNamespace("G",
G, envir="A") in the source R file, either with or without .onLoad
function. Either way, the package was checked and installed free of error. But
after loading, my package still calls the original function G (through F) in
package A, instead of the version I supplied. 
Note that I may do assignInNamespace after my package gets
loaded in a user session, it works perfectly fine. I realize that I may
manually include in my package all the sources for relevant classes and
functions from package A and rewrite function G only. But that?s too awkward
and does not gives credit to package A.
I would really appreciate any thoughts and suggestions on
why assignInNamespace didn?t work in my source package, and how to solve it.
Thanks!
Heyi