pub type PFNGLDEPTHRANGEINDEXEDPROC = Option<unsafe extern "C" fn(index: GLuint, n: GLclampd, f: GLclampd)>;
enum PFNGLDEPTHRANGEINDEXEDPROC { None, Some(unsafe extern "C" fn(_: u32, _: f64, _: f64)), }
No value.
Some value of type T.
T