Things which should be straightforward:

 * The `--sdiff-merge-assist' option is accepted but does nothing.  What
   precisely does it do anyway?

 * Optimise the case of the --brief output style, where a full-blown diff
   isn't necessary but we currently do it.

 * We currently aren't careful about noticing the difference between a
   file which has a return/new line on the last line and one which
   doesn't.  In "robust" output styles, this should result in a warning.
   Admittedly this distinction has never made a difference to any 
   diffing I've done, but if we're going try to be compliant...

 * We do produce the minimal diff (at least if --minimal is enabled we
   do), but we don't produce the "prettiest" diff.  We should post-
   process our diffs to make them prettier.


Things which need a bit more work:

 * Implement --speed-large-files (or at least examine whether or not
   it's worth it to do so).

 * Support diffing of binary files.  Mostly this just requires being
   more careful than we currently are.


Things which would be nice but may/will require a lot of work:

 * Implement a "regexp" module so we can handle more options.

 * Output file dates on context/unified diffs.

 * Directory diffs.