"It seems to me that in 492 U. S. the plurality strains to place a construction on § 188.029 [Footnote 3/2] that enables it to conclude: "[W]e would modify and narrow Roe and succeeding cases," ante at 492 U. S. 521. That statement is ill-advised, because there is no need to modify even slightly the holdings of prior cases in order to uphold § 188.029."