1. Introduction
One crucial task in collaborative environments is finding, combining and filtering information from different (possibly heterogeneous) datasources. Clients in search of information may be unsure which datasources hold the relevant data and it is inconvenient for them to query several datasources separately; in a dynamic system with changing number or location of datasources this situation gets even worse. To facilitate the search process for clients, a new entity called mediator was introduced in such environments (see e.g. [23]). In a mediated system, the client issues a global query to the mediator, the mediator passes partial queries to adequate datasources, retrieves their partial results and returns a global result combined from the partial results to the client (see Figure 1 for a basic mediated system). Moreover, a hierarchy of mediators is possible: a mediator can also serve as a datasource for other mediators. A basic mediated information system