Macro add_scope 

macro_rules! add_scope {
    ($sg:expr, $data:expr, [ $($lbl:expr),* ]) => { ... };
    ($sg:expr, [$($lbl:expr),* ]) => { ... };
    ($sg:expr) => { ... };
}

Creates a scope (with some data if specified), and permission to extend it for each label specified in the label list argument.

TODO: Better documentation, examples.