 The only attempt we know of to address the compositionality worry is Berto and Jago 2019, Chapter 8. They view impossible worlds as sets of sentences (of some ‘worldmaking’ language), where those sentences have a compositional structure. They then argue that, given any semantic content, a grammatical structure can be recovered and analysed in terms of the syntactic structure of the underlying worldmaking sentences. Whether this provides an acceptable notion of compositionality (as they claim) remains to be seen. 