Goal:
  
  A : Type
  t' : A
  P : A → A → Type
  tl : list A
  tl' : list A
  X : OnOne2 P tl tl'
  IHX : ∀ n : nat,
          nth_error tl' n = Some t'
          → ∑ t : A, nth_error tl n = Some t × (t = t') + P t t'
  ============================
   ((t' = t') + P t' t')%type


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := t'
of type
intropattern
id0 := P
of type
intropattern
flags := 
of type
tauto_flags
X2 := A → Type
of type
constr of type tacvalue