∀ a, b. (x0 -1l a) -1l b = (x0 -1l b) -1l a ⊦ ((a0 ' + x0) -1l a) -1l b = ((a0 ' + x0) -1l b) -1l a
Used simplifier rules: ⊦ a + x = a ' + x; (s)