Consider a deductive database with the following rules:
Notice that PARENT(X, Y) means that X and Y are human beings and Y is the (biological) parent of X; ANCESTOR(X, Y) means that Y is the ancestor of X.
Consider the following fact base:
PARENT(john, steve), PARENT(john, olivia), PARENT(olivia, emma), PARENT(olivia, william).
Consider that a database contains the following relations PARENT(X, Y), and a relation BIRTH(X, B), where B is the birth date of a person X.