Macro validated_kernel_approximation 

macro_rules! validated_kernel_approximation {
    (RBF, RandomFourierFeatures, $n:literal) => { ... };
    (Laplacian, RandomFourierFeatures, $n:literal) => { ... };
    (RBF, Nystrom, $n:literal) => { ... };
    (RBF, Fastfood, $n:literal) => { ... };
}

Macro for easy creation of validated kernel approximations