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A clause set is renamable Horn if the result replacing part propositional variable with its complement is a set of Horn clauses. The renamable Horn problem is solvable in linear time, but the maximum renamable Horn problem (MAX-RHS) is NP-hard. In this paper, we present transformations between clause sets and undirected graphs in polynomial time, such that finding a renamable Horn subset of a clause set is equivalent to finding an independent set of vertices of a graph. Then, the problems MAX-RHS and MAX-IND have the same complexity, and MAX-RHS is inapproximable.