Reachability Logic is a formalism that can be used, among others, for expressing partial-correctness properties of transition systems. In this paper we present three proof systems for this formalism, all of which are sound and complete and inherit the coinductive nature of the logic. The proof systems differ, however, in several aspects. First, they use induction and coinduction in different proportions. The second aspect regards compositionality, broadly meaning their ability to prove simpler formulas on smaller systems, and to reuse those formulas as lemmas for more complex formulas on larger systems. The third aspect is the difficulty of their soundness proofs. We show that the more induction a proof system uses, and the more specialised is its use of coinduction (with respect to our problem domain), the more compositional the proof system is, but the more difficult its soundness proof becomes. We also briefly present mechanisations of these results in the Isabelle/HOL and Coq proof assistants.testing of the final running software; but it enables the early catching of errors and the early discovery of key functional-correctness properties, all of which are known to have practical, cost-effective benefits.Contributions. We further study RL on transition systems (TS). We propose three proof systems for RL, and formalise them in the Coq [1] and Isabelle/HOL [10] proof assistants. One may naturally ask: why having several proof systems and proof assistants -why not one of each? The answer is manyfold:• the proof systems we propose have some common features: the soundness and completenes metaproperties, and the coinductiveness nature inherited from RL. However, they do differ in others aspects: (i) the "amount" of induction they contain; (ii) their degree of compositionality (i.e., their ability to prove local formulas on "components" of a TS, and then to use those formulas as lemmas in proofs of global formulas on the TS); and (iii) the difficulty level of their soundness proofs.• we show that the more induction a proof system uses, and the closest its coinduction style to our problem domain of proving reachability-logic formulas, the more compositional the proof system is, but the more difficult its soundness proof. There is a winner: the most compositional proof system of the three, but we found that the other ones exhibit interesting, worth-presenting features as well. • Coq and Isabelle/HOL have different styles of coinduction: Knaster-Tarski style vs. Curry-Howard style. Experiencing this first-hand with the nontrivial examples constituted by proof systems suggested a spinoff project, which amounts to porting some of the features of one proof assistant into the other one. For the moment, porting Knaster-Tarski features into the Curry-Howard coinduction of Coq produced promising results, with possible practical impact for a broader class of Coq users.Related Work. Regarding RL, most papers in the above-given list of references mention its coinductive nature, but do not actually use it. Several Coq mechanisatio...