st: Confusion about fixed effects specification

[Amirhossein Moharreri <amir.h.moharreri@gmail.com>]

I asked this question once but unfortunately no one replied me back
I have got problem in specification of fixed effects in my
estimations. I am using a three-dimensional panel data (bilateral
country flows, commodity, year) in the gravity model. my data forms
like below:
Malaysia to China-Agricultural,year
.
.
.
Malaysia to China-Manufacturing,year
.
.
.
Malaysia to Germany- Agricultural,year
.
.
.
Malaysia to Germany-Manufacturing,year

as you see in this example I will have 4 cross-sections. but the
problem is that I want my model be fixed just by the country pairs but
not by commodity (e.g. I want to estimate the effects of Malaysia to
China not Malaysia to China-agriculture) how can I estimate to get the
required effects?
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/