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


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