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).
 
