def Fin.unpack.step
import bendtorch-core@0.1.0.0/src/shape/core.bend as Coreimport Base
116def Fin.unpack.step(-a: Nat, -b: Nat, rec: Fin(Nat.mul(a, b)) -> Fin(a) & Fin(b), split: Either<Fin(b), Fin(Nat.mul(a, b))>) -> Fin(1n+a) & Fin(b):
117 match split:
118 case Inl{tail}:
119 (FZero{}, tail)
120 case Inr{index}:
121 Fin.unpack.lift(a, b, rec(index))