Function register_dim 

pub fn register_dim<E: TeXEngine>(
    engine: &mut E,
    name: &'static str,
    read: fn(&mut EngineReferences<'_, E::Types>, <E::Types as EngineTypes>::Token) -> TeXResult<<E::Types as EngineTypes>::Dim, E::Types>,
    assign: Option<for<'a, 'b> fn(&'a mut EngineReferences<'b, E::Types>, <E::Types as EngineTypes>::Token, bool) -> TeXResult<(), E::Types>>,
)

Creates a new primitive command that yields (and optionally assigns) a dimension value and registers it with the engine.