Macro fermium::pick

macro_rules! pick {
    ($(if #[cfg($($test:meta),*)] {
      $($if_tokens:tt)*
    })else+ else {
      $($else_tokens:tt)*
    }) => { ... };
    (if #[cfg($($if_meta:meta),*)] {
      $($if_tokens:tt)*
    } $(else if #[cfg($($else_meta:meta),*)] {
      $($else_tokens:tt)*
    })*) => { ... };
    (@__forests [$($not:meta,)*];) => { ... };
    (@__forests [$($not:meta,)*]; [{$($m:meta),*} {$($tokens:tt)*}], $($rest:tt)*) => { ... };
    (@__identity $($tokens:tt)*) => { ... };
}

Does all our conditional compilation selection.