What is the test for severance under the doctrine of severance?

Bearing in mind the respective constitutional positions of parliament and the courts, these excerpts from Schachter v. Canada, 1992 CanLII 74 (SCC), [1992] 2 S.C.R. 679, at paras. 26 and 31 respectively, are apt: Generally speaking, when only a part of a statute or provision violates the Constitution, it is common sense that only the offending portion should be declared to be of no force or effect, and the rest should be spared ... ... Therefore, the doctrine of severance requires that a court define carefully the extent of the inconsistency between the statute in question and the requirements of the Constitution, and then declare inoperative (a) the inconsistent portion, and (b) such part of the remainder of which it cannot be safely assumed that the legislature would have enacted it without the inconsistent portion. Breach of the Principles of Fundamental Justice