Macro define_edge_type 

macro_rules! define_edge_type {
    (
        struct $name:ident $( ( $($visibility:vis $field_ty:ty),* $(,)? ) )?,
        $entity:ident
        $(, $($extra:tt)+),*
    ) => { ... };
    (
        struct $name:ident { $($visibility:vis $field_name:ident : $field_ty:ty),* $(,)? },
        $entity:ident
        $(, $($extra:tt)+),*
    ) => { ... };
    (
        enum $name:ident {
            $($variant:ident),* $(,)?
        },
        $entity:ident
        $(, $($extra:tt)+),*
    ) => { ... };
}