Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  E : Type
  P : E → A → Type
  Q : C → A → Type
  R : B → D → Type
  f : B → C → E
  b : B
  l : list B
  l''' : list D
  hb : All2 R (b :: l) l'''
  x : C
  y : A
  l0 : list C
  l' : list A
  r : Q x y
  ha : All2 Q l0 l'
  hlen : #|b :: l| = #|x :: l0|
  hPQ : ∀ (x : B) (y : C) (z : A) (w : D), R x w → Q y z → P (f x y) z
  IHha : ∀ (l : list B) (l''' : list D),
           All2 R l l''' → #|l| = #|l0| → All2 P (map2 f l l0) l'
  ============================
   (All2 P (map2 f (b :: l) (x :: l0)) (y :: l'))


DEBUG:Going to execute:
noconf hlen
Level 0: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 1: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.