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.
With cxxopts 2.0.0, positional arguments weren’t recognized when other
command-line options were passed before. This has been fixed in the
meantime, but there is no release with the bug fix yet.
This updates cxxopts to a newer commit to ship anthem with this fix.
Boost was only used for program option parsing. To avoid this huge
dependency, this commit replaces boost::program_options with cxxopts,
a header-only library with the same functionality.
cxxopts is added as a submodule, and Boost is removed from the
dependencies in the code and Travis configuration.
With C++17, optionals, an experimental language feature, were moved to
the “std” namespace. This makes C++17 mandatory and drops the now
obsolete “experimental” namespace.
As Travis only provides outdated packages (compilers in particular),
this changes the Travis configuration to use Docker images to build and
test the code. This also has the benefit that multiple distributions can
be tested and not just Ubuntu.
For the time being, Arch Linux and Ubuntu 18.04 are added as supported
platforms.
