Goal:
  
  A : Type
  P : A → Type
  Q : A → Type
  x : A
  l : list A
  p : P x
  X : All P l
  IHX : All (λ x : A, P x → Q x) l → All Q l
  X0 : All (λ x : A, P x → Q x) (x :: l)
  x0 : A
  l0 : list A
  X1 : P x → Q x
  X2 : All (λ x : A, P x → Q x) l
  H0 : x0 = x
  H1 : l0 = l
  ============================
   (Q x)


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := x
of type
intropattern
id0 := P
of type
intropattern
flags := 
of type
tauto_flags
X2 := Type
of type
constr of type tacvalue