| ⊦ sort(x + ins≤(a, y)) = ins≤(a, sort(x + y)), perm(x + ins≤(a, y), a ' + sort(x + y)), perm(x + ins≤(a, y), ins≤(a, sort(x + y))), perm(a ' + sort(x + y), x + ins≤(a, y)) |
⊦ perm(a ' + x, y) ↔ a ∈ y ∧ perm(x, y -1l a)
⊦ sort(x + ins≤(a, y)) = ins≤(a, sort(x + y)), perm(x + ins≤(a, y), a ' + sort(x + y)), perm(x + ins≤(a, y), ins≤(a, sort(x + y))), a ∈ x + ins≤(a, y) ∧ perm(sort(x + y), (x + ins≤(a, y)) -1l a)
Used terms: x + ins≤(a, y), sort(x + y), a