Deque.Deque.lemma.append_assoc: law of bend-kit-collections@0.1.4.1
Import: import bend-kit-collections@0.1.4.1/deque.bend as Deque
Lines 226-232 of bend-kit-collections@0.1.4.1/deque.bend: https://jend.leeeet.dev/read.txt?path=bend-kit-collections@0.1.4.1/deque.bend&start_line=226&end_line=232

import Base

law Deque.lemma.append_assoc:
  for -A: Data
  for  xs: List<&2, A>
  for -ys: List<&2, A>
  for -zs: List<&2, A>
  {List.append(&2, A, List.append(&2, A, xs, ys), zs)
   == List.append(&2, A, xs, List.append(&2, A, ys, zs)) : List<&2, 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