foreign(T, U, Attrs) :-
    foreign_claim(T, U, Attrs),
    forall(member(A, Attrs), (attribute(A, T),
                  attribute(A, U))),
    T\=U.

foreign_key_path(T, T, []) :-
    table(T).

foreign_key_path(T, U, [foreign(T, U, Attrs)]) :-
    foreign(T, U, Attrs).

foreign_key_path(T, V, [foreign(T, U, Attrs) | Rest]) :-
    foreign(T, U, Attrs),
    foreign_key_path(U, V, Rest).

attribute(custkey, customer).
attribute(name, customer).
attribute(address, customer).
attribute(nationkey, customer).
attribute(phone, customer).
attribute(acctbal, customer).
attribute(mktsegment, customer).
attribute(comment, customer).

attribute(nationkey, nation).
attribute(name, nation).
attribute(regionkey, nation).
attribute(comment, nation).

attribute(regionkey, region).
attribute(name, region).
attribute(comment, region).

key_claim([custkey], customer).
foreign_claim(customer, nation, [nationkey]).
foreign_claim(nation, region, [regionkey]).