Goal:
  
  P : E.t -> Prop
  P_Proper : Proper (E.eq ==> impl) P
  s : t
  x : elt
  xs : list elt
  H : Forall P xs -> forall x : elt, InA E.eq x xs -> P x
  H0 : (forall x : elt, InA E.eq x xs -> P x) -> Forall P xs
  H1 : forall x0 : elt, E.eq x0 x \/ InA E.eq x0 xs -> P x0
  x0 : elt
  H2 : InA E.eq x0 xs
  ============================
   (P x0)


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := x0
of type
intropattern
id0 := P
of type
intropattern
flags := 
of type
tauto_flags
X2 := Prop
of type
constr of type tacvalue