Goal:
    A : Type  B : Type  R : nat → A → B → Type  n : nat  b : All2i R n [] []
  ============================
   (let H := CoreTactics.block in
    NoConfusionHom_All2i A B R n [] [] All2i_nil b
    → let H0 := CoreTactics.block in All2i_nil = b)


DEBUG:Going to execute:
fun id =>
             DepElim.generalize_sig id
              ltac:(fun id id' => DepElim.decompose_exists id id')