⊦ prefixn(n, x) = (n ≤ # x ⊃ firstn(n, x);x)
x0 ⊑ x1 ⊦ firstn(# x0, x1) = x0
Used simplifier rules: prefixn: ⊦ prefixn(n, x) = (n ≤ # x ⊃ firstn(n, x);x);% ⊦ # y < # x → ¬ x ⊑ y; (s)% ⊦ m ≤ n ↔ ¬ n < m; (ss)