pub type GEcallback = Option<unsafe extern "C" fn(GEevent, *mut _GEDevDesc, *mut SEXPREC) -> *mut SEXPREC>;
enum GEcallback { None, Some(unsafe extern "C" fn(GEevent, *mut _GEDevDesc, *mut SEXPREC) -> *mut SEXPREC), }
No value.
Some value of type T.
T