1. Prototype-updating apparatus comprising: prototype-issuing means adapted to supply prototype digital objects or prototype items of digital content, each of said prototypes having one or more quantifiable attributes; output means adapted to output prototypes supplied by the prototype-issuing means, for presentation to a user; user-input means adapted to receive input from a user and to discriminate prototype-update commands input by the user; tag input means adapted to register an association between respective prototypes and descriptive tags assigned to said prototypes; a tag model generator adapted to develop models of the relationships between the tags assigned to a prototype and prototype attributes, the tag model generator comprising analysis means adapted to analyse the values of attributes of a plurality of tagged prototypes and the tags assigned to said prototypes; a prototype-classifier adapted, for a specified tag, to quantify the descriptive power of said specified tag in respect of prototypes, said classifier being adapted to apply the tag model developed for the specified tag by the tag model generator; prototype-update-command processing means adapted to identify the prototype-to-be-updated and to identify, in a prototype-update-command received via the user input means, update data indicative of one or more conditions to be met by a set of one or more specified tags of the updated prototype; prototype-update means adapted to select an updated prototype from among candidate prototypes suppliable by the prototype-issuing means, the prototype-update means comprising: similarity-assessment means adapted to assess the degree of global similarity between the prototype-to-be-updated and respective candidate prototypes, and condition-checking means adapted to assess candidate prototypes to determine whether the tags of said candidate prototypes satisfy the conditions specified in the prototype-update command; wherein the prototype-update means is adapted to select as the updated prototype a prototype which the similarity-assessment means has assessed to be globally similar to the prototype-to-be-updated and which the condition-checking means has assessed as having tags that satisfy said conditions; wherein, in response to reception of a prototype-update command, the output means is adapted to output the candidate prototype selected by the prototype-update means.