/* frank and dean are male */
male(frank).
male(dean).

/* ella and judy are female */
female(ella).
female(judy).

/* frank, judy and dean all act */
acts(frank).
acts(judy).
acts(dean).

/* frank, judy, dean and ella all sing */
sings(frank).
sings(judy).
sings(dean).
sings(ella).

/* a person is somebody who is male or female */
person(X) :- male(X).
person(X) :- female(X).

/* an actor is somebody who is male and acts */
actor(X) :- male(X), acts(X).

/* an actress is somebody who is female and acts */
actress(X) :- female(X), acts(X).

/* frank sang with judy, frank sang with dean */
sang_with(frank, judy).
sang_with(frank, dean).