 (C)  does not require the kind of tight correspondence between syntax and semantics we intuitively associate with compositionality. To illustrate this, consider a view, according to which the meaning of a declarative sentence s is the set of possible worlds where s is true. According to such a view, tautologies are synonymous, even though (since Rudolf presumably has some tautological beliefs and lacks others) sentences resulting from embedding tautologies under ‘Rudolf believes that…’ are not. (I also assume a straightforward semantics for propositional attitudes without hidden indexicals or tacit quantification.) Intuitively, this is a violation of compositionality. Still, the semantics is not in conflict with  (C):  tautologies might differ structurally or in the meaning of their constituents, which could explain how their embedding can yield non-synonymous sentences; cf. Carnap 1947 and Lewis 1970.