pub type PFNGLVERTEXSTREAM2DATIPROC = Option<unsafe extern "C" fn(stream: GLenum, x: GLdouble, y: GLdouble)>;
enum PFNGLVERTEXSTREAM2DATIPROC { None, Some(unsafe extern "C" fn(_: u32, _: f64, _: f64)), }
No value.
Some value of type T.
T