open method: prints the solution on the output dimension in a
detailed but “pretty” fashion (language: abbreviated output language
(OL)). Difference to Pretty: if a solution has already been
printed, the next solution is compared with it and only the
new/differing parts are printed. This is useful to find out “leaks”
in the propagation.
close method: “Resets” the functor, i.e., it behaves as if
no solution has previously been printed.