Perm.perm_length: def of bend-mathlib@0.8.1.0
Import: import bend-mathlib@0.8.1.0/perm.bend as Perm
Lines 339-342 of bend-mathlib@0.8.1.0/perm.bend: https://jend.leeeet.dev/read.txt?path=bend-mathlib@0.8.1.0/perm.bend&start_line=339&end_line=342

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

# Permutation preserves length.
def 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}:
  (s, e) = h
  internal_length_eq(A, s, xs, ys, e)

Uses (6):
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List.length
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=Nat
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=Sigma
  https://jend.leeeet.dev/definition.txt?path=bend-mathlib@0.8.1.0/perm.bend&name=internal_length_eq
  https://jend.leeeet.dev/definition.txt?path=bendlib-kernel-list@1.0.0.0/list.bend&name=perm