Goal:
  
  A : Type
  B : Type
  t : A
  P : B → A → A → Type
  b : B
  bs : list B
  hd' : A
  tl : list A
  p : P b t hd'
  e : #|bs| = #|tl|
  ============================
   (Some hd' = Some hd')


DEBUG:Going to execute:
fun tauto_flags X1 X2 id =>
             <coq-core.plugins.tauto::flatten_contravariant_disj@0>