Goal:
    A : Type  P : A → A → Type  hd : A  hd' : A  tl : list A  p : P hd hd'
  ============================
   (P hd hd')


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