def nth_some

bend-collections-laws-crypto@1.0.0.0 · proofs/crypto/argon2/memory.bend:162

import Base
import bend-collections-laws-crypto@1.0.0.0/spec/lib/common.bend as SC
import bend-collections-laws-crypto@1.0.0.0/proofs/lib/nat.bend as N

162def nth_some(+xs: List<&2, U32>, +j: Nat, +h: {Nat.is_lt(j, SC.length(U32, xs)) == True{} : Bool}) -> {SC.nth(U32, xs, j) == Some{nthv(xs, j)} : Maybe<&2, U32>}:
163  match xs j:
164    case Nil{} _:
165      Empty.absurd({SC.nth(U32, Nil{}, j) == Some{nthv(Nil{}, j)} : Maybe<&2, U32>}, N.lt_zero_absurd(j, h))
166    case Con{x, t} 0n:
167      {==}
168    case Con{x, +t} 1n+ +p:
169      nth_some(t, p, h)