left(a,b).
left(c,i).
left(i,j).
right(k,j).

line1:a,b  line2: c,i,j,k.

nextto(X,Y):- left(X,Y);left(Y,X);right(X,Y);right(Y,X).
sameline(X,Y):- nextto(X,Y).
sameline(X,Y):- nextto(X,Z),sameline(Z,Y).

?- sameline(a,c)

ancestor(X,Y) :- parent(X,Y).
ancestor(X,Y) :- parent(X,Z),ancestor(Z,Y).