Goal:
    A : Type  P : A → Type  a : A  X1 : list A  X : All P X1
  ============================
   ([a] = X1
    → let H := CoreTactics.block in let H0 := CoreTactics.block in P a)


DEBUG:Going to execute:
fun p => DepElim.elim_tac ltac:(fun p el => destruct p using el) p