Macro rng 

macro_rules! rng {
    ($start:tt : $end:tt) => { ... };
    ($start:tt : $end:tt : $step:tt) => { ... };
    ($start:tt :) => { ... };
    ($start:tt :: $step:tt) => { ... };
    (: $end:tt) => { ... };
    (: $end:tt : $step:tt) => { ... };
    (:: $step:tt) => { ... };
    (:) => { ... };
}