Macro arg 

macro_rules! arg {
    ($name:expr) => { ... };
    ($name:expr, short: $short:expr) => { ... };
    ($name:expr, required) => { ... };
    ($name:expr, $($key:ident: $value:expr),* $(,)?) => { ... };
    (@set $arg:expr, short: $short:expr) => { ... };
    (@set $arg:expr, help: $help:expr) => { ... };
    (@set $arg:expr, required: $req:expr) => { ... };
    (@set $arg:expr, takes_value: $tv:expr) => { ... };
    (@set $arg:expr, default: $def:expr) => { ... };
}

Macro helper for argument definition