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% the raw data in this example are books, some of them sold with extras, and
% with some of extras weighting enough for their weight to be declared:

% book(Name, Author, Year)
book('The Philosopher''s Stone', 'J. K. Rowling', 1997).
book('The Chamber of Secrets',   'J. K. Rowling', 1998).
book('The Prisoner of Azkaban',  'J. K. Rowling', 1999).
book('The Goblet of Fire',       'J. K. Rowling', 2000).
book('The Order of the Phoenix', 'J. K. Rowling', 2003).
book('The Half-Blood Prince',    'J. K. Rowling', 2005).
book('The Deathly Hallows',      'J. K. Rowling', 2007).

% extra(Book, Kind)
extra('The Philosopher''s Stone', quidditch_set).
extra('The Chamber of Secrets',   map).
extra('The Half-Blood Prince',    audio_cd).
extra('The Deathly Hallows',      horcrux_set).

% weight(Extra, Weight)
weight(quidditch_set, 2000).
weight(horcrux_set,   1000).