“…The close relationship between Bray's work and that of Penner has also been shown. 22 It was observed in the numerical treatment of the expansion involving a single recombination reaction that the flow could often be described by a near-equilibrium portion, a short transition region, followed by chemically frozen flow. Bray suggested a sudden-freezing approximation, in which a point is chosen to divide the flow into equilibrium and frozen portions.…”