JESS: Unruly rules

(deftemplate dataset
    (slot name)
    (slot nobs (default 0))
    (slot nvars (default 0))
    (multislot variables)
    (multislot variableTypes)
    (slot submitEditDatasetForm)
    (slot has-errors (default FALSE))
    (slot isFree (default TRUE)))

(deftemplate subscription-prices
    (slot maxDataElements)
    (slot monthlyPrice)
    (slot yearlyPrice))

(deffacts initial-facts
    (nextPage nil)
    (R nil)
    (subscription-prices (monthlyPrice 25) (yearlyPrice 100) (maxDataElements 500)))

(defrule set-dataset-isFree
    (page "show-dataset.xslt")
    ?x <- (dataset (nobs ?) (nvars ?))
    ?y <- (subscription-prices (maxDataElements ?))
=>
    (if (> (* ?x.nobs ?x.nvars) ?y.maxDataElements) then
        (modify ?x
                (isFree FALSE))))

This works but seems to me to defeat the purpose of using a rule engine. Is this where predicate constraints are used?
I did the following and it appears to work:

(defrule set-dataset-isFree
    (page "show-dataset.xslt")
    (dataset (nobs ?nobs))
    ?x <- (dataset (nvars ?nvars))
    (subscription-prices (maxDataElements ?y&: (< ?y (* ?nobs ?nvars))))
=>
    (modify ?x (isFree FALSE)))

Am I going down the right path here? It looks like I'll be doing a lot of this.

Donald Paul Winston
[email protected]