⊦ m ≤ n ↔ (∃ n0. m + n0 = n)
n < m, n0 < m0, ∃ n1. (m + m0) + n1 = n + n0 + 1 ⊦ m = 0, m0 = 0
Used terms: n + n0 + 1, m + m0