Perm.internal_swap_head_length: def of bend-mathlib@0.8.1.0
Import: import bend-mathlib@0.8.1.0/perm.bend as Perm
Lines 305-314 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=305&end_line=314

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

def internal_swap_head_length(-A: Data, xs: List<&2, A>) -> {List.length(&2, A, K.swap_head(A, xs)) == List.length(&2, A, xs) : Nat}:
  match xs:
    case Nil{}:
      {==}
    case +x <> t:
      match t:
        case Nil{}:
          {==}
        case +y <> u:
          {==}

Uses (4):
  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=bendlib-kernel-list@1.0.0.0/list.bend&name=swap_head