Keith, I had some trouble following your [solution to puzzle 95](https://forum.azimuthproject.org/discussion/comment/18634/#Comment_18634), because I'm a tad confused about how you defined \$$d\$$.
Could you please expand on this?
From the current exposition, it seems that \$$d : \mathbf{Cost} \times \mathbf{Cost} \to \mathbf{Cost}\$$; is that right?