Goal:
  
  A : Type
  P : A → Type
  x : A
  l : list A
  p : ∥ P x ∥
  X : All (λ x : A, ∥ P x ∥) l
  ============================
   (All P l → ∥ All P (x :: l) ∥)


DEBUG:Going to execute:
<coq-core.plugins.ltac::intro@1> $1