pub type PFNGLSECONDARYCOLOR3DPROC = Option<unsafe extern "C" fn(red: GLdouble, green: GLdouble, blue: GLdouble)>;
pub enum PFNGLSECONDARYCOLOR3DPROC { None, Some(unsafe extern "C" fn(f64, f64, f64)), }
No value.
Some value of type T.
T