/* upper case used for variables, lower case for constants.
   Terminate each clause by a period.  A clause can be written
   on multiple lines. Do not leave any blank space. */

/* these are rules */

parent(X,Y):-mother(X,Y).
parent(X,Y):-father(X,Y).
grandparent(X,Y):-parent(X,Z),parent(Z,Y).

/* and these facts */
mother(sonja,mary).
mother(sonja,jane).
father(john,jim).
father(john,bob).
father(bob,bill).
father(bob,dan).

/* and these are goals 

Find all X that are grandparents of Y.  Type ; after the first answer
is returned to see more answers.

grandparent(X,Y).

Find who John is a granparent of

grandparent(john,X).
*/