⊦ m < n ↔ (∃ m0. m + m0 = n ∧ m0 ≠ 0)
n < m, ∃ m1. n0 + m1 = m0 ∧ m1 ≠ 0, m + m0 ≤ n + n0 ⊦ m = 0, m0 = 0
Used terms: m0, n0