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

Lemma proper_sym_arrow_iffT_2 :
forall `(Symmetric A R)`(Symmetric B R')`(Proper _ (R==>R'==>arrow) f),
Proper (R==>R'==>iffT) f.