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 in order 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 input program:
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.
patrick