Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  Q : B → A → Type
  b : B
  bs : list B
  hd : A
  tl : list A
  a : All2 Q (b :: bs) (hd :: tl)
  hd' : A
  p : P b hd hd'
  e : #|bs| = #|tl|
  ============================
   (#|bs| = #|tl|)


DEBUG:Going to execute:
inv a
Level 1: evaluation returns
inversion_clear H
where
H := a
of type
ident of type tacvalue