chapter 7 exercise 7-7: cross-reference, differ line and lines in the output

This commit is based on the one before previous commit.
Here is how you can do that in GitHub:
1. select previous commit.
2. click revert on the upper-right corner of GitHub window. That will
create a new local commit that revert previous commit, and the result of
that commit is of course the one before previous commit.
3. select that new local commit, click rollback. That could dismiss the
local commit, make it as changes in your source code only