sequence test = {{"UK","London"},
                 {"US","New York"},
                 {"US","Birmingham"},
                 {"UK","Birmingham"}}

---------------------
-- probably stable --
---------------------
function cmp(object a, object b)
    return compare(a[1],b[1])
end function
test = custom_sort(routine_id("cmp"),test)
pp(test,{pp_Nest,1})

-----------------------
-- guaranteed stable --
-----------------------
function tag_cmp(integer i, integer j)
    return compare({test[i][1],i},{test[j][1],j})
--  return compare(test[i][1],test[j][1])
end function
sequence tags = custom_sort(routine_id("tag_cmp"),shuffle(tagset(4)))
for i=1 to length(tags) do
    ?test[tags[i]]
end for