Macro zamm_yang::add_flag

macro_rules! add_flag {
    ($name:ident, $owner:ident, $doc:expr, $dual_doc:expr) => { ... };
    ($name:ident <= $parent:expr, $owner:ident, $doc:expr, $dual_doc:expr) => { ... };
}

Defines a new flag, and add it as a property of the owner.