# y0 = (# y < m ⊃ # y;m), firstn(m, y0 + z0) = y + z, m ≤ # y0 + # z0 ⊦ y = y0
# y0 = (# y < m ⊃ # y;m), y0 + z0 = y + z ⊦ y = y0, m ≤ # y0 + # z0