Goal:
  
  A : Type
  P : A → A → Prop
  a : A
  l : list A
  a0 : A
  l' : list A
  r : A
  r' : A
  H : Forall2 P (a :: l ++ [r]) (a0 :: l' ++ [r'])
  H3 : P a a0
  H5 : Forall2 P (l ++ [r]) (l' ++ [r'])
  H0 : Forall2 P l l'
  H1 : P r r'
  ============================
   (P r r')


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