$ export HGMERGE=merge
$ echo 'XXX this is broken and must be fixed'
XXX this is broken and must be fixed
$ hg merge
merging letter.txt
merge: warning: conflicts during merge
merging letter.txt failed!
0 files updated, 0 files merged, 0 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges
$ cat letter.txt
Greetings!
<<<<<<< /tmp/tour-merge-conflictBrdfE0/scam-merge/letter.txt
I am Shehu Musa Abacha, cousin to the former
=======
I am Alhaji Abba Abacha, son of the former
>>>>>>> /tmp/letter.txt~other.1ctEYy
Nigerian dictator Sani Abacha.