Brockett, 472 U. S., at 504-505 (invalidating obscenity statute only to the extent that word lust was actually or effectively excised from statute); United States v. Grace, 461 U.S. 171, 180-183 (1983) (invalidating federal statute banning expressive displays only insofar as it extended to public sidewalks when clear line could be drawn between sidewalks and other grounds that comported with con