and
′) such that
apart from the assumption of the theorem which we want to prove.
48.7 Cases (iv) and (v) of [48.4] deal with the comparability of time-measurements in different time-systems. The same remarks as those in [48.6] apply; namely that the method of [48.5] could be applied, if independently we could convince ourselves that the requisite pair,
and
′ (one a point-track and one a rect), exist.