Quantifying Over Trees in Monadic Second-Order Logic

Monadic Second-Order Logic (MSO) extends First-Order Logic (FO) with variables ranging over sets and quantifications over those variables. We introduce and study Monadic Tree Logic (MTL), a fragment of MSO interpreted on infinite-tree models, where the sets over which the variables range are arbitrary subtrees of the original model. We analyse the expressiveness of MTL compared with variants of MSO and MPL, namely MSO with quantifications over paths. We also discuss the connections with temporal logics, by providing non-trivial fragments of the Graded µ-CALCULUS that can be embedded into MTL and by showing that MTL is enough to encode temporal logics for reasoning about strategies with FO-definable goals.