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


DEBUG:Going to execute:
fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]