Macro trace 

macro_rules! trace {
    () => { ... };
    ($val:expr) => { ... };
    ($val:expr $(,)?) => { ... };
    ($($val:expr),+ $(,)?) => { ... };
}

Define C-like debug trace macro: trace!().

use dmacro::*;
fn foo(n: usize) {
    trace!(format!("n = {}", n));

    // ...
    trace!("ur lucky!");
}

foo(20080512usize);