st: Creating a Group Pair ID (where the generating variables order shouldn't matter)

[J Taylor <jwtayl@gmail.com>]

I am trying to create an ID corresponding to numbers from two lists.
For example, if the two lists were of countries, one would have

clear
input str20 c1id str20 c2id
"US" "Canada"
"US" "Mexico"
"Canada" "US"
"US" "France"
"France" "England"
"France" "US"
end
egen newid = group(c1id c2id)

I would like newid to create an ID pair for each country pair.  My
first instinct was to use the egen group command.  However, the
problem is that egen group takes into account which id comes first.
For example, (c1id,c2id)=(United States,Canada) and (c1id,c2id)=(
Canada ,United States) have different IDs.  I would like them to be
able to have the same ID.  That is, I would like to create newid as a
group pair ID, reflecting which two countries are in the pair, and
where the order doesn't matter.

Any thoughts on how to do this?

Thanks,
JT
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