Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  b : B
  bs : list B
  hd : A
  hd' : A
  tl : list A
  p : P b hd hd'
  e : #|bs| = #|tl|
  ============================
   (P b hd hd' × hd :: tl = [] ++ hd :: tl ∧ hd' :: tl = [] ++ hd' :: tl)


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