⊦ a ∈ s ↔ (∃ s0. s = s0 ++ a ∧ ¬ a ∈ s0)
# s < 3, c ∈ s, b ∈ s, ∃ s0. s = s0 ++ a ∧ ¬ a ∈ s0 ⊦ a = b, a = c, b = c
Used terms: s, a