⊦ a ∈ st -- b ↔ a ≠ b ∧ a ∈ st
∀ a, st. a ∈ st \ s0 ↔ a ∈ st ∧ ¬ a ∈ s0 ⊦ a ≠ a0 ∧ a ∈ st \ s0 ↔ a ∈ st ∧ a ≠ a0 ∧ ¬ a ∈ s0
Used terms: a0, st \ s0, a