List.length_map: law of bend-mathlib@0.8.1.0
Import: import bend-mathlib@0.8.1.0/list.bend as List
Lines 178-184 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=178&end_line=184

import Base

# Mapping preserves the length.
law length_map:
  for ~A: Type
  for ~B: Type
  for ~f: A -> B
  for xs: List<A>
  {List.length(&1, B, List.map(~A, ~B, ~f, xs)) == List.length(&1, A, xs) : Nat}

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=List.map
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=Nat