detect if token in macro present, branch inside macro

macro_branch.rs

/// # Exmaple
/// ```
/// select! {
///     { $($yes)? } { if_present() } { if_absent() }
/// }
macro_rules! select {
    ({ $($_:tt)+ } { $($keep:tt)* } { $($discard:tt)* } ) => { $($keep)* };
    ({} { $($discard:tt)* } { $($keep:tt)* } ) => { $($keep)* };
}

//...

macro_rules! void {
  ($($_:tt)*) => {}
}

macro_rules optional_arg {
    ($($yes:tt)?) => {{
        //... regular code
        $({
            void!($yes)
            // some coniditonal code
        })?
        //... more regular code
    }}
}