We now consider an identity reaction r i , with its reactants and products being identical. This reaction is contained in an idealised set of reactions A, such that the set of reverse reactions A-1 is equivalent to A. Such reactions may be labelled isodromic, as the reverse reactions show the same features as the forward reactions, stemming from how the reactants form products of similar nature. It follows that (𝐴 ~ 𝐴 -1 ∧ ∃ r 𝑖 ∈ 𝐴 | r 𝑖 = r -1 𝑖 ) ⟹ 𝑓 𝐴 = 𝑓 𝐴,-1 . Thus, the kinetic-thermodynamic relationship of the set of forward reactions and backward isodromic reactions, where an identity reaction belongs by definition to both sets, must be modelled by the same function as by definition they show the same energy-related features. The identity reaction r i is, by definition, thermoneutral, and its reverse is itself, and so 𝑓 𝑆 (∆𝐸 𝑟 (r 𝑖 )) = 𝑓 𝑆,-1 (∆𝐸 𝑟 (r 𝑖 )) for any idealised set containing r 𝑖 . Following Equation , 𝑓′ 𝐴 (0) = 0.5 and is symmetric about (0, 0.5). 17