Concatenate gpx files

1. mv $firstfile head.gpx
2. 
   sed  -e '1,/<trkseg>/d' -e '/<\/trkseg>/,$ d' -i 20*
3. Verify:
   for a in * ; do if grep trkseg $a ; then echo $a has trkseg; fi; done
4. 
   for a in 20* ; do cat $a >> output.gpx; done