(** ** Recursion operator *)
Definition C := .\ "x", "y"; "y" (.\ "z"; "x" "x" "y" "z").
Definition rho s : term := Eval cbv in .\ "x" ; C C !s "x".

Lemma rho_correct u v : proc u -> proc v -> rho u v ≻^3 u (rho u) v.