/* To analyze the family structure of the family of Queen Victoria.
English friend of mine notes there wasn't a Harry. I put him in.

Answers the compelling question: Who is X the sister of?

Ask: ?-sisterof( alice, X ). or ?-sisterof( alice, harry ).

  or ?-sisterof( alice, X ), loves( X, wine ). as an example of a
  complex question. 

or even:

sisterof( alice, X ), loves( X, wine ), loves( alice, wine ).

*/

sisterof( X, Y ) :- parents( X, M, F ), 
                    female( X ),
                    parents( Y, M, F ).

parents( edward, victoria, albert ).
parents( harry, victoria, albert ).
parents( alice, victoria, albert ).
female( alice ).

loves( harry, wine ).
loves( alice, wine ).