List.append_assoc: law of bend-mathlib@0.8.1.0
Import: import bend-mathlib@0.8.1.0/list.bend as List
Lines 32-39 of bend-mathlib@0.8.1.0/list.bend: https://jend.leeeet.dev/read.txt?path=bend-mathlib@0.8.1.0/list.bend&start_line=32&end_line=39

import Base

# Append is associative: (xs ++ ys) ++ zs = xs ++ (ys ++ zs).
law append_assoc:
  for -a: Quant
  for -A: Kind(a)
  for xs: List<a, A>
  for -ys: List<a, A>
  for -zs: List<a, A>
  {List.append(a, A, List.append(a, A, xs, ys), zs) == List.append(a, A, xs, List.append(a, A, ys, zs)) : List<a, A>}

Uses (2):
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List.append