The code responsible for completing formulas made the assumption that
all head variables could be safely removed from the list of free
variables of each formula. This is only correct given the current
limitation that only rules with singleton heads are supported.
Because of this assumption, code with multiple elements in the head were
completed to an incorrect result instead of issuing an error that such
rules aren’t supported yet.
This commit improves the code by excluding only variables that are
actually replaced from the list of free variables and not all head
variables. Still, other places will need to be adjusted for full support
of rules with multiple elements in the head. For this reason, this also
adds an error message indicating that only rules with singleton heads
are supported as of now.
Finally, multiple test cases are added to check that the supported
features related to the issues outlined above are translated without
exceptions, while errors are returned when attempting to use unsupported
features.
This refactoring separates predicates from their declarations. The
purpose of this is to avoid duplicating properties specific to the
predicate declaration and not its occurrences in the program.
This adds support for declaring predicates as placeholders through the
“#external” directive in the input language of clingo.
Placeholders are not subject to completion. This prevents predicates
that represent instance-specific facts from being assumed as universally
false by default negation when translating an encoding.
This stretches clingo’s usual syntax a bit to make the implementation
lightweight. In order to declare a predicate with a specific arity as a
placeholder, the following statement needs to be added to the program:
#external <predicate name>(<arity>).
Multiple unit tests cover cases where placeholders are used or not as
well as a more complex graph coloring example.
