xxdiff, a graphical file comparator and merge tool
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xxdiff is a file comparator and merge tool. It is an open-source
replacement for Rudy Wortel's xdiff which comes with SGI machines 
(which was itself inspired by SGI's gdiff).  xxdiff can be used to 
graphically display differences between two (eventually three) files.
It can also be used to merge two files together, by allowing the user
to select regions and save the results of the selection.  It is also
an aid in code reviewing, as it allows a reviewer to look at the pieces 
of changed code and save selected portions of the code to an editor to
make comments (horizontal diffs make reviewing much easier, especially
for a long list of files).