Martin (1992: 324):
Accordingly, as far as lexical relations between activities are concerned, a distinction will be drawn between expectancy and implicational relations according to whether one activity is probably followed by another (modalisation; activity a probably followed by activity b) or is absolutely determined by the other (modulation; activity a necessarily followed by activity b) as in the scientific explanation discussed above.
 There are two inconsistencies here. The first is the notion that lexical relations (lexicogrammatical stratum) can obtain between activities (construed as context). The second is the notion that relations between activities — in SFL theory: between figures in sequences — can be lexical rather than semantic (realised grammatically).
 There is a metafunctional inconsistency here. In this chapter on experiential semantics, interpersonal relations (modality) are proposed for (lexical) relations between activities (misconstrued as context).
 This is a category error. Implication is not necessity; necessity is a hyponym of implication. In logic, necessity contrasts with sufficiency as types of implicational relations between statements.