| ⊦ # filter(rmdup(x), y) ≤ # y |
# y < # filter(rmdup(x), y) ⊦ filter(rmdup(x), y) = []
Used simplifier rules: l: ⊦ # x = 0 ↔ x = []; from specification list-data%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic1%lels-01: ⊦ ¬ m ≤ n ↔ n < m; from specification nat