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


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := b
of type
intropattern
id0 := P
of type
intropattern
flags := 
of type
tauto_flags
X2 := A → A → Type
of type
constr of type tacvalue