Merge in Source Control

In theory, this could be very difficult:

* What happens if Jane changed some of the same lines that Joe changed, but in different ways?
* What happens if Jane's changes are functionally incompatible with Joe's?
* What happens if Jane made a change to a C# function which Joe has deleted?
* What happens if Jane changed 80 percent of the lines in the file?
* What happens if Jane and Joe each changed 80 percent of the lines in the file, but each did so for entirely different reasons?
* What happens if Jane's intent was not clear and she cannot be reached to ask questions?