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'
    × (t = hd') + (∑ i' : B, Some b = Some i' × P i' t hd'))


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