⊦ dups(x) ∧ ¬ dups(y) → ¬ perm(x, y)
Used simplifier rules: app-05: ⊦ dups(x) → dups(x + y); from specification list-dup%a: ⊦ (x + y) + z = x + y + z; from specification list Used terms: x, y + z