⊦ prefixn(n, x) = (n ≤ # x ⊃ firstn(n, x);x)
a ∈ (n0 ≤ # x ⊃ firstn(n0, x);x), n0 ≤ n1 ⊦ a ∈ (n1 ≤ # x ⊃ firstn(n1, x);x)
Used simplifier rules: prefixn: ⊦ prefixn(n, x) = (n ≤ # x ⊃ firstn(n, x);x);