Macro if_empty 

macro_rules! if_empty {
    { { }                  { $($x:tt)* } $({ $($y:tt)* })? } => { ... };
    { { $($nonempty:tt)+ } { $($x:tt)* } $({ $($y:tt)* })? } => { ... };
}

Helper for assisting with defining macros that need to expand conditionally when an argument is empty.

if_empty!{ {   } { x } { y } } // => x
if_empty!{ { z } { x } { y } } // => y
// etc.

Note: The { y } argument may be omitted.