rate-methods
signature(object = "ExpParameter"): returns the slot rate of the parameter of the distribution
signature(object = "ExpParameter"): modifies the slot rate of the parameter of the distribution
signature(object = "Exp"): returns the slot rate of the parameter of the distribution
signature(object = "Exp"): modifies the slot rate of the parameter of the distribution
signature(object = "DExp"): returns the slot rate of the parameter of the distribution
signature(object = "DExp"): modifies the slot rate of the parameter of the distribution