1. A formal verification method comprising the steps of: with a formal verification tool, performing a partial search of a state space that represents the behavior or a system model, the partial search performed by inputting, at a plurality of states of the state machine, only a subset of a complete set of inputs, said complete set of inputs comprising every possible input that the system-model inputs can possibly assume, when the system model is in operation, in every possible sequence; and monitoring the behavior of the system model as a result of inputting said subset of the complete set of inputs.