∀ b0. b0 ∈ s → b0 < b, ∀ b0. b0 = a ∨ b0 ∈ s → b0 < b ⊦ a < b
a < b, ∀ b0. b0 ∈ s → b0 < b ⊦ ∀ b0. b0 = a ∨ b0 ∈ s → b0 < b