Intentionally left implicit.

Rule - user-level class encapsulating WCCL rule.
Action - internal class for Rule's actions
ActionExecContext - helper class for execution of actions.

That forced antlr to run every time. The workaround is to remove the output directory
so files are generated from clean slate, thus have proper time stamps.

It is coherent now with the SentenceContext::is_inside(int abs_pos)
- SentenceContext::is_inside(Position pos) is added, while
Position::is_inside(SentenceContext) is removed

I.E. empty sentence shouldn't be passed anyway and that is what
Operator class is checking for. But otherwise, at the level of
operator, it can deal with a sentence that has current pos outside
of boundaries.

Previously it would mandate agreement was met on all categories
in a tag with enough categories, if such tag was found.
Now it checks for agreement only on the number of categories that
actually match, including the case of zero categories matching
(the latter does meet the weak agreement).