What is the test for reducing the compensation of an incumbent Judge below the amount he was entitled to when the legislation was passed?

It might also be of some benefit to reiterate that, in the case at bar, we are not concerned merely with a reduction of the compensation of an incumbent Judge below the amount which he was receiving immediately before the legislation was passed, but, a reduction below the amount to which he was entitled and was in fact receiving from the moment of his appointment. Furthermore, the case at bar does not fall within the ambit of those decisions such as Judges v. A.-G. Sask., 1937 CanLII 368 (UK JCPC), [1937] 2 D.L.R. 209, [1937] 1 W.W.R. 508, [1937] W.N. 109, or in Re Constitutional Questions Act; Re Income Tax Act, 1932, 1936 CanLII 154 (SK CA), [1936] 4 D.L.R. 134, [1936] 2 W.W.R. 443, which establish that the members of the judiciary are not, by reason of their constitutional position, exempt at law from the provisions of general taxing statutes applicable to all citizens of the country.