def internal_swap_at_length

bend-mathlib@0.8.1.0 · perm.bend:316

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

316def internal_swap_at_length(-A: Data, +i: Nat, +xs: List<&2, A>) -> {List.length(&2, A, K.swap_at(A, i, xs)) == List.length(&2, A, xs) : Nat}:
317  match i:
318    case 0n:
319      internal_swap_head_length(A, xs)
320    case 1n+p:
321      match xs:
322        case Nil{}:
323          {==}
324        case +x <> t:
325          %Equal.sym(Nat, List.length(&2, A, K.swap_at(A, p, t)), List.length(&2, A, t), internal_swap_at_length(A, p, t)) : {1n+_ == 1n+List.length(&2, A, t) : Nat}
326          {==}