M M1 M2
AB
|`-------v M2 clones M
aB AB file A is change in mainline
|`---v AB' file B is changed in M2
| aB / | M1 clones M
| ab/ | M1 changes B
| ab' | M1 merges from M2, changes to B conflict
| | A'B' M2 changes A
`---+--.|
| a'B' M2 merges from mainline, changes to A conflict
`--.|
??? depending on which ancestor we choose, we will have to redo A hand-merge, B hand-merge, or both but if we look at the files independently, everything is fine