message: small comment
diff: @@ -20,6 +20,8 @@ use std::fmt::Debug;
  ///
  /// [Example](https://github.com/argmin-rs/argmin/blob/master/examples/lbfgs.rs)
  ///
  +/// TODO: Implement compact representation of BFGS updating (Nocedal/Wright p.230)
  +///
  /// # References:
  ///
  /// [0] Jorge Nocedal and Stephen J. Wright (2006). Numerical Optimization.
  