% From The Art of Eyepl, Chapter 16 — Sockets: naming the knowledge boundary.
socket(family_source, provides(predicate(parent, 2))).
plug(family_file, family_source).

parent(pat, jan).
parent(jan, emma).

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