def perm_length

bend-mathlib@0.8.1.0 · perm.bend:339

import Base
import bendlib-kernel-list@1.0.0.0/list.bend as K

339# Permutation preserves length.
340def perm_length(-A: Data, +xs: List<&2, A>, -ys: List<&2, A>, h: K.perm(A, xs, ys)) -> {List.length(&2, A, ys) == List.length(&2, A, xs) : Nat}:
341  (s, e) = h
342  internal_length_eq(A, s, xs, ys, e)