1. Expert system for solving problems on the basis of a flow of input messages concerning the operation and the status of a network of machines while meeting real time constraints, said status changing with time, said system comprising: filter means for detecting in realtime significant events on the basis of input messages, each significant event being a manifestation of a problem, as judged by a human expert; a database for storing: all input messages, the significant events detected by the filter means, intermediate data produced by the expert system, a model of the network of machines, and archives relating to problems processed previously, whether solved or not; reasoning means for reasoning responsive to said filter means and to said database, said reasoning means comprising: correlating means for determining whether a significant event detected in realtime by said filter means can be correlated with significant events previously detected by said filter means to thereby establish a new or updated common problem associated with a plurality of interrelated significant events having a possible common cause; a plurality of specialists for processing part of the common problem, each comprising a subset of rules and knowledge provided by a human expert and being capable of executing in parallel without blocking each other; and command means responsive to each significant event detected in realtime by said filter means and also responsive to each new or updated common problem associated with a plurality of interrelated significant events established by said correlating means for selecting and executing specialists according to heuristic rules provided by a human expert and making a decision or declaring that a decision is impossible at the end of a predetermined time period in respect of a problem represented by significant events communicated to said reasoning means by said filter means; a graphical interface for human/expert system communications; and expert assistant means for supplying a human expert with data stored in said database.