Merged. Thanks for the review, Gabi!

I've added a draft implementation on the issue171 branch on
https://bitbucket.org/malte/downward. Gabi, does it look OK to you? (Still have
to do more tests.)

See also issue446.

See also msg1881 and the testcase that goes with issue413.

I will contact the organizers and let them know.

The domain is buggy. It uses "suit" both as the name of a type and as the name
of a binary predicate. According to the PDDL spec, types can also be used as
unary predicates, so there's a conflict between "suit" being a predicate of
arities 1 and 2. I guess this should be reported to the conference organizers.
Erez, can you do this? (Feel free to CC me if more explanations should be
necessary -- I think this is discussed in the original PDDL document by Drew
McDermott.)

If you want, we can try to
1) support this even though it's not legal, or
2) improve the error message,
but I don't think this should have a high priority.

The translator fails on a freecell problem provided by IPC organizers (attached)

The error message:

Traceback (most recent call last):
  File "downward/FD/src/translate/translate.py", line 590, in <module>
    sas_task = pddl_to_sas(task)
  File "downward/FD/src/translate/translate.py", line 423, in pddl_to_sas
    relaxed_reachable, atoms, actions, axioms = instantiate.explore(task)
  File 
"/IBM_STORAGE/USERS_DATA/batman/downward/FD/src/translate/instantiate.py", line 
67, in explore
    model = build_model.compute_model(prog)
  File 
"/IBM_STORAGE/USERS_DATA/batman/downward/FD/src/translate/build_model.py", line 
325, in compute_model
    rule.fire(next_atom, cond_index, queue.push)
  File "/storage12/batman/python/lib/python2.6/contextlib.py", line 34, in 
__exit__
    self.gen.throw(type, value, traceback)
  File "/IBM_STORAGE/USERS_DATA/batman/downward/FD/src/translate/timers.py", 
line 32, in timing
    yield
  File 
"/IBM_STORAGE/USERS_DATA/batman/downward/FD/src/translate/build_model.py", line 
324, in compute_model
    rule.update_index(next_atom, cond_index)
  File 
"/IBM_STORAGE/USERS_DATA/batman/downward/FD/src/translate/build_model.py", line 
96, in update_index
    for position in self.common_var_positions[cond_index]]
IndexError: tuple index out of range