Proof with eauto with wd.
intros.
inversion Harg; subst;
eapply extcall_arg_valid in H; eauto.
eapply extcall_arg_valid in H0; eauto...
Qed.

Lemma extcall_arguments_valid:
forall f m ef args rs
(Hexternal: extcall_arguments rs m (ef_sig ef) args)
(Hrs_wd: regset_wd f rs)
(Hmem_wd : valid_mem m)
(Hobs_eq: domain_memren f m),
valid_val_list f args.