Macro enso_shapely::_cartesian_impl

macro_rules! _cartesian_impl {
    ([[$f:path]] $out:tt [] $b:tt $init_b:tt) => { ... };
    ([[$f:path] $args:tt] $out:tt [] $b:tt $init_b:tt) => { ... };
    ($f:tt $out:tt [$a:tt $($at:tt)*] [] $init_b:tt) => { ... };
    ($f:tt [$($out:tt)*] [$a:tt $($at:tt)*] [$b:tt $($bt:tt)*] $init_b:tt) => { ... };
}

Internal helper for cartesian macro.