Goal:
  
  A : Type
  n : nat
  t' : A
  P : nat → A → A → Type
  i : nat
  hd : A
  hd' : A
  tl : list A
  p : P i hd hd'
  H : nth_error tl n = Some t'
  ============================
   ((t' = t') + P (i + S n)%nat t' t')%type


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