A method, apparatus and product for generating elements based on generation streams. The method comprises: obtaining one or more generation streams, wherein the streams comprise elements, wherein each element is a formal specification of an operation that stimulates a system, wherein based on each of the generation streams one or more alternative stimuli for the system can be generated, which stimuli comprises operations according to the elements; and generating a stimuli in accordance with the one or more generation streams, wherein the stimuli comprises at least one hybrid operation, wherein the hybrid operation complies simultaneously with two or more elements of the one or more generation stream, whereby the stimuli is comprised of a number of operations that is smaller than a sum of the numbers of elements of the one or more generation streams.