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Full Idea
In S5 there are six modalities: no-operator; necessarily; and possibly (and their negations). In any sequence of operators we may delete all but the last to gain an equivalent formula.
Gist of Idea
S5 has just six modalities, and all strings can be reduced to those
Source
Rod Girle (Modal Logics and Philosophy [2000], 3.5)
Book Ref
Girle,Rod: 'Modal Logics and Philosophy' [Acumen 2000], p.46
A Reaction
Such drastic simplification seems attractive. Is there really no difference, though, between 'necessarily-possibly', 'possibly-possibly' and just 'possibly'? Could p be contingently possible in this world, and necessarily possible in another?