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


DEBUG:Going to execute:
<coq-core.plugins.ltac::unify@0> $1 $2
DEBUG LTAC Evaluated term: (Type -> Type)
DEBUG LTAC Evaluated term: (nat -> A -> Type)