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


DEBUG:Going to execute:
fun => <coq-core.plugins.ltac::assumption@0>