Function register_expandable 

pub fn register_expandable<E: TeXEngine>(
    engine: &mut E,
    name: &'static str,
    f: fn(&mut EngineReferences<'_, E::Types>, &mut Vec<<E::Types as EngineTypes>::Token>, <E::Types as EngineTypes>::Token) -> TeXResult<(), E::Types>,
)

Creates a new expandable primitive and registers it with the engine.