st: gravity model - country pair identifier necessary?

[John Bunge <jota.be@web.de>]

Dear statalisters,

I will estimate a panel gravity model with countries.

Is it sufficient for (i) tsset and (ii) estimation to have each part of the pair identified or do I need to include an extra identifier for each pair?

To make my query clear, I have countries A, B and C, which are identified by these letters. 
My pairs are AB, AC and BC, which I might identify by these letter combinations or by a numerical identifier (e.g. 1, 2, 3).

Any suggestions are highly appreciated.

Thanks,

John.
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