1. A computer program product for operating a computer, said computer program product comprising: a computer readable medium; first program instruction means for instructing a computer processor to form a sequence in which a plurality of event handling programs or parts of the same program can receive notification of an event; second program instruction means for instructing a computer processor to notify a first one of the programs or parts in the sequence of the occurrence of the event; third program instruction means for instructing a computer processor to receive an indication from said first program or part whether or not the event notification should be propagated to the next program or part in the sequence; fourth program instruction means for instructing a computer processor to propagate the event notification to said next program or part in the sequence if the indication is to propagate the event notification; and fifth program instruction means for instructing a computer processor to block propagation of the event notification to said next program or part in the sequence if the indication is not to propagate the event notification; and wherein each of said program instruction means is recorded on said medium in executable form and is loadable into a computer memory for execution by the associated processor to form part of an operating system.