Goal:
  
  A : Type
  R : A → A → Type
  a : A
  l : list A
  IHl : All2 R l l → All (λ x : A, R x x) l
  X : All2 R (a :: l) (a :: l)
  x : A
  y : A
  l0 : list A
  l' : list A
  X0 : R a a
  X1 : All2 R l l
  H0 : x = a
  H1 : l0 = l
  H : y = a
  H3 : l' = l
  ============================
   (R a a)


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