Macro step_dbg 

macro_rules! step_dbg {
    (bg=$bg:expr, fg=$fg:expr, length=$length:expr, $($arg:expr),* $(,)?) => { ... };
    (bg=$bg:expr, fg=$fg:expr, $($arg:expr),* $(,)?) => { ... };
    (fg=$fg:expr, $($arg:expr),* $(,)?) => { ... };
    ($($arg:expr),* $(,)?) => { ... };
    () => { ... };
}

colorfully steps through code debugging given expressions