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.introduce p;
              DepElim.elim_case p || destruct p || (case p; clear p)