Macro featurizers 

macro_rules! featurizers {
    ($a:expr) => { ... };
    ($a:expr $(, $tail:expr)*) => { ... };
}

A macro for general composition of featurizers. featurizers!(ftzr_a, ftzr_b, ... ftzr_z) will featurize input data exactly z times. Must use Merged in the output.

let my_ftzr = featurizers!(ftzr1,
                           ftzr2,
                           ftzr3,
                           ftzr_n);
let feats: HashSet<Merged<&str>> = my_ftzr.featurize(your_data);