As long as there is only one

<sep> <sep>

pair

perl -lne'print unless$s;push@s,$_ if$s;if(/<sep>/){if($s){foreach(sor
+t @s){print}$s=0}else{$s+=1}}'
[download]

This is a bit quick and dirty as the last tag is included in the sort (no prob if the items to be sorted are all numeric)

Cheers,
R.