List.length_append: law of bend-mathlib@0.8.1.0
Import: import bend-mathlib@0.8.1.0/list.bend as List
Lines 49-55 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=49&end_line=55

import Base

# The length of an append is the sum of the lengths.
law length_append:
  for -a: Quant
  for -A: Kind(a)
  for xs: List<a, A>
  for -ys: List<a, A>
  {List.length(a, A, List.append(a, A, xs, ys)) == Nat.add(List.length(a, A, xs), List.length(a, A, ys)) : Nat}

Uses (5):
  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
  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=Nat.add