How do I get mdiff to produce usable output?

Accordinng to the man mdiff page, just executing

mdiff abitree wptree

should give me some kind of list of similarities and differences between
the files abitree and wptree, much as

diff abitree wptree

does. But instead I get nothing. Why don't I just use diff, you ask?
Because when I go into production, I'll have a lot more than two files to
compare, the situation that mdiff was apparently designed for.

I can get a word-by-word dofference instead of a line-by-line difference
if I specify

mdiff -W abitree wptree

but that's not what I want, because words aren't unique enough for the
files to synch properlym and the output is gibberish.

I'm running a frequently updated wheezy system here.

hendrik@notlookedfor:~/dv/mtxt/dev$ mdiff --version
mdiff (GNU wdiff) 0.6.5

Copyright (C) 1992, 1997, 1998, 1999, 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.

Written by Franc,ois Pinard <pinard@iro.umontreal.ca>.
hendrik@notlookedfor:~/dv/mtxt/dev$ uname -a
Linux notlookedfor 3.0.0-1-686-pae #1 SMP Sat Aug 27 16:41:03 UTC 2011
i686 GNU/Linux
hendrik@notlookedfor:~/dv/mtxt/dev$






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