∀ i, j. i + j + i1 = (i + j) + i1 ⊦ i + j + i1 = (i + j) + i1
Used simplifier rules: add-pred: ⊦ i + j -1 = (i + j)-1;% ⊦ i -1 = j -1 ↔ i = j; (s)