⊦ (x + y) -l a = (x -l a) + (y -l a)
∀ a. rev(x0) -l a = rev(x0 -l a) ⊦ a = a0, (rev(x0) -l a) + (a0 ' -l a) = rev(x0 -l a) + a0 '
Used terms: a, a0 ', rev(x0)