Complex sentence: Gelenbe's G-network model was the first to show that this is not the case, by introducting "negative customers" which can destroy or eliminate other customers, and he thus generalised the theory of product form networks.
Split into simpler sentences: Gelenbe's G-network model was the first to show that this is not the case. By introducing "negative customers" which can destroy or eliminate other customers, he generalised the family of product form networks.