Theorem switch_subs : forall (P : proc) (n1 n2 : name) (p : PP) (x : VV), pname p <> n1 -> vname x <> n2 -> subs_var_proc (subs_par_proc P n2 p) n1 x = subs_par_proc (subs_var_proc P n1 x) n2 p.