1. A method comprising: storing a plurality of media sets in a knowledge base of media sets; and preprocessing the knowledge base to quantify an association between pairs of media items in the knowledge base, comprising: for each item in a mediaset, identifying every other item in the same mediaset, so as to define all of the pairs of media items in that mediaset; repeating said identifying step for every other mediaset in the knowledge base, thereby defining every pair of items that appears in any mediaset in the knowledge base; and for each identified pair of media items, adjusting a concurrency metric value responsive to each additional occurrence of the same pair of items in the knowledge base.