Goal:
    A : Type  B : Type  C : Type  R : A → B → C → Type  b : All3 R [] [] []
  ============================
   (let H := CoreTactics.block in
    NoConfusionHom_All3 A B C R [] [] [] All3_nil b
    → let H0 := CoreTactics.block in All3_nil = b)


DEBUG:Going to execute:
fun id cont =>
             DepElim.generalize_sig_gen id
              ltac:(fun id id' id'1 t =>
                      try rename id into id',id' into id; cont id'1 id)