Classical logics for strategic reasoning, such as Coalition Logic and Alternating-time Temporal Logic, formalize absolute strategic reasoning about the unconditional strategic abilities of agents to achieve their goals. Goranko and Ju introduced a logic ConStR for strategic reasoning about conditional strategic abilities. However, its completeness is still an open problem. ConStR has three featured operators, and one of them has the following reading: For some action of A that guarantees the achievement of her goal, B has an action to guarantee the achievement of his goal. The logic about this operator is called CConStR. In this paper, we prove completeness for two fragments of CConStR. The key notions of our proof approach include downward validity lemma, grafted models, and upward derivability lemma. The proof approach has good potential to be applied to the completeness of ConStR and other logics.

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