24 Jan 2008: in the r_solver, the impose_neg_constraints opens a choice point, while in the fd_solver it uses constraints. Request: improve the r_solver version.

9 jun 2008: Implications with false in the head could be implemented more efficiently. E.g.: a(1) -> false abducing a(X) opens a choice point (case analysis), in which one branh fails. One could directly impose non_unify(X,1).