Goal:
    A : Type  P : A → Type  a : A  X : All P [a]
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in P a)


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)