Macro channel 

macro_rules! channel {
    ($expr:expr) => { ... };
    ($expr:expr, label = $label:expr) => { ... };
    ($expr:expr, capacity = $capacity:expr) => { ... };
    ($expr:expr, label = $label:expr, capacity = $capacity:expr) => { ... };
    ($expr:expr, capacity = $capacity:expr, label = $label:expr) => { ... };
    ($expr:expr, log = true) => { ... };
    ($expr:expr, label = $label:expr, log = true) => { ... };
    ($expr:expr, log = true, label = $label:expr) => { ... };
    ($expr:expr, capacity = $capacity:expr, log = true) => { ... };
    ($expr:expr, log = true, capacity = $capacity:expr) => { ... };
    ($expr:expr, label = $label:expr, capacity = $capacity:expr, log = true) => { ... };
    ($expr:expr, label = $label:expr, log = true, capacity = $capacity:expr) => { ... };
    ($expr:expr, capacity = $capacity:expr, label = $label:expr, log = true) => { ... };
    ($expr:expr, capacity = $capacity:expr, log = true, label = $label:expr) => { ... };
    ($expr:expr, log = true, label = $label:expr, capacity = $capacity:expr) => { ... };
    ($expr:expr, log = true, capacity = $capacity:expr, label = $label:expr) => { ... };
}