Goal:
  
  P : E.t -> Prop
  P_Proper : Proper (E.eq ==> impl) P
  s : t
  x : elt
  xs : list elt
  H2 : P x
  H3 : forall x : elt, InA E.eq x xs -> P x
  H1 : Forall P xs
  H0 : forall x : elt, InA E.eq x xs -> P x
  x0 : elt
  H4 : InA E.eq x0 xs
  ============================
   (P x0)


DEBUG:Going to execute:
t_reduce