Common use of Lemma 2 Clause in Contracts

Lemma 2. Let k = [k1, k2, k3, k4] be a key agreement scheme. Let α, β, γ, δ, δ′, η, η′, φ be functions, such that K1(a, b) = δ(k1(α(a), β(b))) K2(b, c) = η(k2(β(b), γ(c))) K3(a, e) = φ(k3(α(a), η′(e))) K4(d, c) = φ(k4(δ′(d), γ(c))). are well-defined. If δ′(δ(x)) = x and η′(η(x)) = x for all x in the appropriate domains, then K = [K1, K2, K3, K4] is a key agreement scheme.