1. A computer-readable medium having encoded thereon instructions for causing a computer system to: receive a definition of a propagation block having an input port for receiving information indicative of an input attribute-set of an input signal, and an output port for providing information defining an output attribute-set of an output signal, and receive a definition of a rule-set for determining said output attribute-set on the basis of said input attribute-set; and provide a user-interface for enabling a user to define said rule-set.