(** When the relation on the domain is symmetric, a predicate is compatible with [iff] as soon as it is compatible with [impl]. Same with a binary relation. *)

Lemma proper_sym_impl_iff_2 :
forall `(Symmetric A R)`(Symmetric B R')`(Proper _ (R==>R'==>impl) f),
Proper (R==>R'==>iff) f.