dups(x) ⊦ ∃ y, a, z. x = y + a ' + z ∧ a ∈ y ∧ ¬ dups(y)
Used simplifier rules: in-04-01: ⊦ a ∈ x → dups(x + a ' + y);%a: ⊦ (x + y) + z = x + y + z; from specification list