Function register_box 

pub fn register_box<E: TeXEngine>(
    engine: &mut E,
    name: &'static str,
    read: fn(&mut EngineReferences<'_, E::Types>, <E::Types as EngineTypes>::Token) -> TeXResult<Either<Option<TeXBox<E::Types>>, BoxInfo<E::Types>>, E::Types>,
)

Creates a new primitive command that yields a box and registers it with the engine.