Oxokohlenstoffe und verwandte Verbindungen, 31 [1]. 3-Antipyryl-4-hydroxy-cyclobutendion (Antipyrylsemiquadratsäure) und seine Derivate / Oxocarbons and Related Compounds, 31 [1]. 3-Antipyryl-4-hydroxy-cyclobutenedione (Antipyrylsemisquaric Acid) and its Derivatives