Perm.internal_swap_head_invol: def of bend-mathlib@0.8.1.0
Import: import bend-mathlib@0.8.1.0/perm.bend as Perm
Lines 256-265 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=256&end_line=265

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

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

Uses (2):
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List
  https://jend.leeeet.dev/definition.txt?path=bendlib-kernel-list@1.0.0.0/list.bend&name=swap_head