Accepted paper at LICS’20 Apr 16, 2020 Bartosz Bednarczyk, Stéphane Demri, Raúl Fervari, and Alessio Mansutti. “Modal Logics with Composition on Finite Forests: Expressivity and Complexity”, accepted at 35th Annual ACM/IEEE Symposium on Logic in Computer Science (LICS 2020).