:- use_module(library(chr)).
handler view_update.

constraints father/2, mother/2.

father(X,Y), father(Z,Y) ==> Z=X.
mother(X,Y), mother(Z,Y) ==> Z=X.

grandfather(X,Z):- father(X,Y), father(Y,Z).
grandfather(X,Z):- father(X,Y), mother(Y,Z).

current_db:-
    father(peter, paul),
    father(paul,jens),
    mother(marie,jens).