Goal:
  
  A : Type
  P : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  h : OnOne2 P tl tl'
  x : A
  y : A
  u : list A
  v : list A
  a : P x y
  H : tl = u ++ x :: v
  H0 : tl' = u ++ y :: v
  ============================
   (hd :: tl = (hd :: u) ++ x :: v)


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