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 : BinInt.Z.of_nat #|bs| = BinInt.Z.of_nat #|tl|
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|tl|)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|bs|)
  ============================
   (Some b = Some b × P b t hd')


DEBUG:Going to execute:
idtac