[Haskell-cafe] Question, re: illegal instance error.

Hi all, I don’t understand why this code: 50: instance (IsNat n, RealFloat a) => FFT (RTree n) a where is generating this error: fft_test.hs:50:36: Illegal instance declaration for ‘FFT (RTree n) a’ (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use FlexibleInstances if you want to disable this.) In the instance declaration for ‘FFT (RTree n) a’ and was hoping someone could help me get it. Thanks! -db