A generative testing framework is one where the code itself generates test cases.
Typically you write code to generate test cases according to one or more assumptions you would like to test.
I'm not fambiliar with mbunit itself, but for example using the Clojure generative test framework test.generative you can write tests like:
(defspec integers-closed-over-addition
(fn [a b] (+' a b)) ;; input fn
[^long a ^long b] ;; input spec
(assert (integer? %))) ;; 0 or more validator forms
This test directly specifies the assumption you want to test (i.e. that the addition of two longs always results in an integer).
The important point is that you don't have to specify particular long
vales for testing - the framework itself will generate arbitrary combinations of inputs and check that your assertions hold true in every case.