1. A prototype-updating apparatus comprising: a microprocessor configured to implement: a prototype-issuing unit configured to supply prototype digital objects or prototype items of digital content, each of said prototypes having one or more quantifiable attributes; an output unit configured to output prototypes supplied by the prototype-issuing unit, for presentation to a user; a user-input unit configured to receive input from a user and to discriminate prototype-update commands input by the user; a tag input unit configured to register an association between respective prototypes and descriptive tags assigned to said prototypes; a tag model generator configured to develop models of the relationships between the tags assigned to a prototype and prototype attributes, the tag model generator comprising an analysis unit configured to analyze the values of attributes of a plurality of tagged prototypes and the tags assigned to said prototypes; a prototype-update-command processing unit configured to identify the prototype-to-be-updated and to identify update data in a prototype-update-command received via the user input unit, wherein the update data identifies a set of one or more specified tags and defines a degree to which the specified tags describe the updated prototype for output in response to the prototype-update-command; a prototype-update unit configured to select, from among candidate prototypes supplied by the prototype-issuing unit, the updated prototype for output in response to the prototype-update-command, the prototype-update unit further comprising: a similarity-assessment unit configured to assess the degree of global similarity between the prototype-to-be-updated and respective candidate prototypes, a prototype-classifier configured, for each of the tags specified in the prototype-update command, to quantify the descriptive power of said specified tag for respective candidate prototypes, said classifier being configured to quantify said descriptive power by applying the tag model developed for the specified tag by the tag model generator, and a condition-checking unit configured to assess candidate prototypes to determine, based on the descriptive power values quantified by the prototype-classifier, whether the tags specified in the update data describe the candidate prototypes to the degree defined by the prototype-update command; wherein the prototype-update unit is further configured to select, as the updated prototype, a prototype which the similarity-assessment unit has assessed to be globally similar to the prototype-to-be-updated and which the condition-checking unit has assessed as being described, to the degree defined by the prototype-update command, by the tags specified in the update data; wherein, in response to reception of a prototype-update command, the output unit is further configured to output the candidate prototype selected by the prototype-update unit.