macro_rules! helper {
    ($name:ident, Write)        => {mut $name};
    ($name:ident, WriteExpect)  => {mut $name};
    ($name:ident, WriteStorage) => {mut $name};
    ($name:ident, Read)         => {$name};
    ($name:ident, ReadExpect)   => {$name};
    ($name:ident, ReadStorage)  => {$name};
}

macro_rules! system {
    ($name:ident = $($cname:ident <- $typ:ident<$life:tt,$ctype:ty>;)* $es:block) => {
        struct $name;
        impl<'a> System<'a> for $name {
            type SystemData = ($( $typ<$life,$ctype>, )*);

            fn run(&mut self, ($( helper!($cname, $typ),)*): Self::SystemData)
            {
                $es
            }
        }
    };
}