⊦ chain(Sn) ↔ (∀ m, n. Sn(m) ⊆ Sn(n) ∨ Sn(n) ⊆ Sn(m))
f(S0)(x), S0 ⊆ S ⊦ ∀ m, n. (m = 0 ⊃ S0;S) ⊆ (n = 0 ⊃ S0;S) ∨ (n = 0 ⊃ S0;S) ⊆ (m = 0 ⊃ S0;S), f(S)(x)
Used terms: λ n. (n = 0 ⊃ S0;S)