Goal:
  
  A : Type
  a : A
  l : list A
  n : nat
  P : A → bool
  d : A
  IHl : ∀ n : nat, forallb P l → n < #|l| → ∃ x : A, nth n l d = x ∧ P x
  H : is_true (P a && forallb P l)
  H0 : S n < S #|l|
  ============================
   (∃ x : A, nth n l d = x ∧ P x)


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