Alarmingly, the five justices agreeing that the Town of Greece had not violated the Constitution may well have done much more than find the Town's practices analogous to those of Nebraska previously upheld in Marsh. . . . [T]he deeper problem is not whether government sponsored prayer passes Justice Kennedy's test, but rather the Court's embrace of a constitutional standard that depends chiefly on the justices' perception of governmental practice as permissible at the time constitutional language now invoked to challenge that practice was framed.