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In 1980, the writer proposed an instantaneous nonlinearity model with variable knee sharpness. In 1996, Litva and Lo found that it gave incorrect results for a two-tone intermodulation test, and in 2000, Loyka diagnosed the reason: nonanalyticity. Further study showed that no problem exists with typical real-world signals; nevertheless, this paper presents a new model that eliminates the problem entirely. For context, it also discusses envelope models and their application to radio-frequency power amplifiers.