# from files
./symmetrize.sh input
# from stdin
cat input | ./symmetrize.sh
1 1
1 2
3 4
1 1
1 2
2 1
3 4
4 3
awk '{ print $0 "\n" $2 "\t" $1 }' $@ | sort | awk 'p != $0 { print $0 } { p = $0 }' |