Goal:
  
  T : Type -> Type
  M : Monad T
  A : Type
  P : nat -> A -> Type
  k : nat
  a : A
  l : list A
  f : forall (n : nat) (x : A),
      nth_error (a :: l) n = Some x -> T (∥ P (k + n) x ∥)
  X : ∥ P (k + 0) a ∥
  Y : ∥ Alli P (S k) l ∥
  ============================
   (∥ Alli P k (a :: l) ∥)


DEBUG:Going to execute:
<coq-core.plugins.ltac::unify@0> $1 $2
DEBUG LTAC Evaluated term: A
DEBUG LTAC Evaluated term: (forall (n : nat) (x : A),
                            nth_error (a :: l) n = Some x ->
                            T (∥ P (k + n) x ∥))