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Progressive rock elements appear in other metal subgenres. Black metal is conceptual by definition, due to its prominent theme of questioning the values of Christianity. Its guttural vocals are sometimes used by bands who can be classified as progressive, such as Mastodon, Mudvayne and Opeth. Symphonic metal is an extension of the tendency towards orchestral passages in early progressive rock. Progressive rock has also served as a key inspiration for genres such as post-rock, post-metal and avant-garde metal, math rock, power metal and neo-classical metal.