Goal:
  
  A : Type
  t : A
  P : A → A → Type
  hd' : A
  tl : list A
  p : P t hd'
  X : A → Type
  ============================
   (Some hd' = Some hd' × (t = hd') + P t hd')


DEBUG:Going to execute:
<coq-core.plugins.ltac::clear@0> $1