| ⊦ n ≤ # y ∧ ¬ dups(x + y) → ¬ dups(x + firstn(n, y)) |
dups(x + firstn(n, y)), n ≤ # y ⊦ dups(x + y), dups(x), dups(y)
Used simplifier rules: a: ⊦ (x + y) + z = x + y + z; from specification list%a: ⊦ ¬ dups(x + y) → ¬ dups(x) ∧ ¬ dups(y); (forward) from specification list-dup