1. A method for calculating a description of potential responses of a distributed system in response to at least one input stimulus, the distributed system including nodes whose operation is described by respective node behavior descriptions, the method comprising: applying symbolic evaluation to plural node behavior descriptions so as to produce plural response descriptions that describe responses of nodes; combining at least the plural response descriptions so as to arrive at the description of potential responses of the distributed system; and in response to an input requirement that is input from a client, outputting a description of at least one potential response of the distributed system; wherein at least some of the node behavior descriptions are incomplete or inaccurate.