Information gathering for the user is usually a task which is embedded within the context of some other task. Its cost structure and value are therefore dependent on the embedding task which may force changes over time. For instance, the value of the information and the relevance of specific items are not intrinsic properties of the information objects (e.g. report documents) but depend on the assessment made from the point of view of the embedding task. For instance, product development may be overtaken by events in the wider market. causing a different direction to be taken. Users, therefore, quire ever more flexible methods to change and refine their searches.