(Sorry, accidentally changed the status. Changing back to resolved.)

Thanks for fixing! If you ask me though, allowing zero-ary functions to be
written without parentheses is a "feature" that should be removed from the
language, rather than supported...

Fixed and merged.

True, that would be a sufficient solution. I will take care for this.

> To catch such an invalid PDDL input, we would need the information about the
> language (predicate and function symbols) from the domain description when
> parsing the task specification. While this is not really a problem, I would
> not want to do it for this issue in isolation but rather make this a subtask of
> issue215.

Would it not be enough to forbid (= ...) where the last argument is not a number?

I checked the PDDL specification. Actually, this cannot be checked on a purely
syntactic basis because an expression like (= total-cost 0) is allowed in the
init section (the additional parentheses in "(= (total-cost) 0)" are not 
required for 0-ary functions). However, specifying the equality of objects in
the initial state is not allowed according to the PDDL specification.

To catch such an invalid PDDL input, we would need the information about the
language (predicate and function symbols) from the domain description when
parsing the task specification. While this is not really a problem, I would
not want to do it for this issue in isolation but rather make this a subtask of 
issue215.

There is some great intelligence in the test whether an element is contained in
a python set. This makes it almost impossible to reproduce this bug.
Nevertheless I attached a small task that covers the critical scenario. If you
really want to see the problem then add a line "init_facts = list(init_facts)"
to Atom.instantiate in line 325 of pddl/conditions.py:

        init_facts = list(init_facts)
        if atom in fluent_facts:
            result.append(atom)
        elif atom not in init_facts:
            raise Impossible()

The problem is indeed the declaration of equal objects in the initial state
specification. The translator assumes that all expressions with an = in the
:init section are assignments to fluents. This should be easy to fix but I
cannot reproduce the behavior with Patrik's example task. So I will try to make
a smaller problem task first.

On attached problem, I sometimes get the following translator crash:

[...]
Completing instantiation... Traceback (most recent call last):
  File "/home/path/pkg/fd-clean/src/translate/translate.py", line 665, in <module>
    main()
  File "/home/path/pkg/fd-clean/src/translate/translate.py", line 655, in main
    sas_task = pddl_to_sas(task)
  File "/home/path/pkg/fd-clean/src/translate/translate.py", line 478, in
pddl_to_sas
    reachable_action_params) = instantiate.explore(task)
  File "/home/path/pkg/fd-clean/src/translate/instantiate.py", line 77, in explore
    return instantiate(task, model)
  File "/home/path/pkg/fd-clean/src/translate/instantiate.py", line 57, in
instantiate
    fluent_facts, type_to_objects)
  File "/home/path/pkg/fd-clean/src/translate/pddl/actions.py", line 117, in
instantiate
    objects_by_type, effects)
  File "/home/path/pkg/fd-clean/src/translate/pddl/effects.py", line 129, in
instantiate
    self._instantiate(var_mapping, init_facts, fluent_facts, result)
  File "/home/path/pkg/fd-clean/src/translate/pddl/effects.py", line 135, in
_instantiate
    self.condition.instantiate(var_mapping, init_facts, fluent_facts, condition)
  File "/home/path/pkg/fd-clean/src/translate/pddl/conditions.py", line 327, in
instantiate
    elif atom not in init_facts:
  File "/home/path/pkg/fd-clean/src/translate/pddl/conditions.py", line 284, in
__eq__
    return (self.hash == other.hash and
AttributeError: 'Assign' object has no attribute 'hash'

But only sometimes: Running the translator again, with the same input, works
fine and produces a normal output. Repeating a few times, it seems I get the
crash about once in four tries.

Unfortunately, the files are a bit big, but this is the smallest instance on
which I have observed this behaviour.

This occurs on two different machines, one that has python "2.7.1+" and one that
has "2.7.3rc2".

One thing to note: The initial state spec contains "(= p1 p1) (= p2 p2) ..."
etc, i.e., explicit declarations of all(?) objects that are equal. Not sure if
that's an error or not. (Well, according to PDDL it's most likely an error; but
question is whether the translator accepts this as normal.)