Full Idea
Sidelle defends conventionalism against a posteriori necessities by 'factoring' a necessary a posteriori truth into an analytic component and a nonmodal component. The modal force then comes from the analytic part, and the a posteriority from the other.
Gist of Idea
Necessary a posteriori is conventional for necessity and nonmodal for a posteriority
Source
report of Alan Sidelle (Necessity, Essence and Individuation [1989]) by Theodore Sider - Writing the Book of the World 12.8
Book Reference
Sider,Theodore: 'Writing the Book of the World' [OUP 2011], p.282
A Reaction
[I note that Sidelle refers, it seems, to the nonmodal component as a 'deep explanatory feature', which is exactly what I take an essence to be].