#15 Support for stable semantics

[Hannes Strass]

Implement the stable operator associated to the characteristic operator (Denecker, Marek and Truszczynski, 2004).

Make the usual semantics independent of the concrete operator that is used, and allow them to be used with the approximating as well as with the stable operator. The technical problem here is that in each call of the stable operator, the approximating operator is called a linear number of times.

Add a command line argument -s --stable to use the stable operator instead of the approximating one. So for example, the following arguments then compute:

-s -mod ... two-valued stable models
-s -com ... three-valued stable models
-s -prf ... M-stable models
-s -sem ... L-stable models